Gloves of Haste works the same way as Chainmail but for DPS instead of survivability. On higher star levels heroes receive more damage, but their attack speed stays constant. Because of this, the attack speed increase is extremely valuable in the late game. Moreover, mana gain per hit is capped, so attack speed is in general more efficient at increasing a hero’s mana gain than flat damage (the hero will hit more often, thus gaining mana more often).

The reason Chainmail is S-Tier is that it scales extremely well into the late game, unlike most Tier One items. The armor provides a percentage-based physical damage reduction. Units scale in terms of health with star-levels, but their armor is mostly constant, so a bonus of 10 armor is good even in the very late game. In general, given equal HP, putting Chainmail on the lower-armor heroes will be more efficient (unless you are running a comp with clear "main tanks" and you need to buff their EHP with priority).

S-tier for the same reason as Chainmail and Gloves - it scales very well into the late game. Most powerful in Mage strategies, but very good in almost all cases because most strats have at least one hero with significant ability damage output (mostly AoE).

Like chainmail, it scales well into the late game and provides great EHP against physical damage (although unlike armor, only attack damage, not physical damage coming from abilities). It's most efficient on heroes who already have high HP and armor.

A very high impact carry item when you put it on your main damage dealer while keeping other Blood-Bound heroes on the frontline. A second such item could easily get you a first place in the lobby. If you get it too late in the game and you don’t already run Ogre/Warlock, it might be better to skip picking it because it’s hard to adjust your lineup to account for the Bloodbound bonus right away, especially if you have a mature lineup short on board space. Picking it too late and waiting for Warlock/Ogre to show up in the shop while having to level-up for the extra slot can force you to hold a zero-impact item for a long time, which is counter-productive.

Great on heroes who have trouble getting max Mana fast enough. The best example is Keeper of the Light because of his low attack damage, but there are plenty other heroes who use Void Stone well.

Morbid Mask: "+30% lifesteal."
Great on frontline heroes who deal high damage - Lycan, Terrorblade, etc. Not that useful on any other kind of hero, and the problem is that it most likely gets replaced in the late game because those heroes are your main carreis and you want a high-tier item on them.

In general, HP regen is more useful on tanky lineups that live for longer so that it has more time to heal and give value. In the late game the HP regen falls off sharply, but the magic resistance starts becoming more valuable. Hood is best used on tanky warriors – they already have high HP and armor, so the magic resistance gives them a significant effective HP boost in the late game. Moreover, they die slowly early on, so they get value from the regen.

Headdress: "Units within 1 cell of the equipped hero gain +20 health per second."
A lot of AoE health regen means it's great on tanky lineups that take damage together - Knights, Brute, Brawny, Warriors, Scrappy, etc. The big downside is that health regen falls of sharply in the late game.

Claymore: “+30 Attack Damage.”
bottom of the Tier 1 list because it’s the opposite of Gloves of Haste – a flat damage increase doesn’t scale in the late game well because heroes tend to get plenty of damage when they get a higher star level. Moreover, mana per hit is capped, so on upgraded units, it doesn’t help with mana gain as well. That said, it’s pretty good in the very early game.

Vitality Booster: “+250 Health.”
Same logic as for Claymore – it provides a good survivability boost in the early game, but the flat HP increase doesn’t scale well into the late game because heroes receive a lot of HP with upgrades anyway. Like Claymore, it’s pretty decent in the early game.

Great on any attack-damage carry which is very likely to accumulate kills like e.g. Ember. Outright imba on Deadeye heroes if you get it from early on - Bloodseeker.

Another very well-scaling item on a hero who is likely to get kills. With Cloak you are mostly restricted to frontline heroes who already deal good-enough damage, like Lycan, Lifestealer, and Terrorblade.

Better than Chainmail if the unit is taking hits from multiple enemies. This makes it best on tanks that take a lot of focus fire – e.g. if your frontline consists of just a few units as in a Mages/Hunters/Assassins draft, then Vanguard can provide a lot of value. If your frontline is larger and the damage is spread-out (as in Warriors and Knights), Vanguard is less valuable.

Desolator: "Attack target loses 5 Armor for 5 seconds."
Good on heroes with good single-target physical damage output - mostly Hunters and Assassins. Great if you have other sources of armor reduction - Heartless, Alchemist, Slardar.

Crystalys: "+25 Attack Damage. 20% chance to Critical Hit for 200% damage."
Even though attack speed is more powerful than flat damage in Underlords, Crystalys is quite strong with it's crit. Great on Hunteres who have multiple chances to proc it.

Not game-breaking, but very versatile. It’s a free frontline unit with instant crowd control. There are very, very few situations in which Target Buddy isn’t useful. Moreover, its impact skyrockets on lineups that have frontline problems (Hunters, Mages, even Assassins).

Could be strong on lineups that like box positioning in the corner and lack enough frontline heroes (Hunters and Mages). It’s mostly useful against Assassins to prevent them from reaching your most-important heroes (Sniper/KotL/etc.). That said, in all other cases it’s very hard to use efficiently and feels pretty underwhelming. Be careful not to block-off your own units outside of their attack range.

Arcane Boots: “When equipped hero has greater than 50% mana, restore 25 mana to all allies 2 cell away. (once per battle)”
Pretty self-explanatory – the helps your units cast their spells faster. It has a pretty decent range, so it’s more valuable on lineups that want to stay together. On the frontline, it will trigger faster and allow your CC units to cast their spells before the enemy CC units. On the backline, it will help slow mana-generating heroes like KotL cast their spells in a more reasonable time, so it’s quite possible to put on the backline as well. Try to put it on a hero with a lower mana pool – putting it on KotL itself means that the Boots won’t trigger for a very long time, while Batrider can activate it very fast even though he’s not on the frontline. 

Blink Dagger: “At the start of battle, teleport behind enemy lines to the enemy farthest away from the equipped hero and gain 50 mana.”
Situationally very powerful but picking it can also be a mistake simply because often you end up with a lineup that doesn’t have great Dagger carriers. Some decisions about Dagger are also not clear-cut: in theory, a two-star Tide can benefit a lot from a Dagger because he will cast Ravage on top of the enemy damage-dealing backline. At the same time, this means you are removing Tide (a great tank) from the frontline, decreasing the EHP of your frontline. If your Blink Tide is your last unit alive, this is a very bad sign and you should probably remove Dagger from him – you want him to tank damage before your damage dealers.

Armlet of Mordiggian: "+90 Attack Damage. +10 Armor. Equipped hero loses 100 health per second."
The stats are great, but the drawback is pretty obvious - 100 DPS loss is significant. More useful on lineups with good sustain (Warlocks) to mitigate the health loss.

Dragon Lance: "RANGED ONLY. +20 Attack Damage. Equipped hero has unlimited range."
Good on short-range ranged physical damage heroes like Luna, Medusa, etc. That said, it clashes a great deal with high-tier carry items, which means it's not particularly useful in the late game.

Mantle of the Flayed Twins: "Equipped hero is a Deadeye."
Very decent on long-ranged damage-dealers (Shadow Fiend, most Hunters). The big drawback is that it clashes for the item slot with more powerful carry items later on in the game and might become useless.

Force Staff: “When the equipped hero takes damage from a melee attack, push the attacker up to 6 cells away.”
Could be situationally useful to counter Assassins (which is very situational) OR to prevent an enemy frontline hero from gaining mana fast enough (with great positioning you can e.g. Force the enemy Tidehunter away from the enemy frontline. That said, the item is extremely unreliable and feels useless very often, making it one of the lowest priority items in the game.

Orb of Venom: "Enemies damaged by the equipped hero's attacks are Poisoned for 35 DPS."
Falls off very fast as the damage is not significant. If there are further "poisoned" effect synergies introduced further down the road, it could improve, but currently, it's simply an underwhelming item.

Quelling Blade: "+25 Attack Damage. Equipped hero will prefer to attack summoned units. Attacks against summoned units crit for 200% damage."
Obviously useful versus Summoner strategies, but extremely weak if you're not facing a Summoner build, which makes the item extremely situational.

Silver Edge: “+30 Attack Speed. +300 Health. Whenever equipped hero attacks a unit, Break icon.png break that unit for 5 seconds.”
Break is an incredibly powerful mechanic in Underlords, as it disables most cumulative bonuses (Stonehall Pike and Cloak, etc.). Moreover, it provides good stats besides the brek effect.

Mask of Madness: “Hero is silenced. 15% Lifesteal. +60 Attack Speed.”
One of the best items in the game on carries with passive abilities (Slark, Draw Ranger, Troll Warlord). The obvious drawback is the silence – it makes the item sub-optimal on most other heroes because they have valuable active spells. (There are some exceptions where the silence is actually useful because the hero does more DPS with attacks rather than with his ult, most notably two or three-star Sniper).

The benefit is that on a very tanky unit it can deal very good DPS. On a Brawny unit with plenty of HP (ideally Axe who also has a Taunt), it can even kill certain squishy enemy damage-dealers before they kill the Blade Mail carrier. A drawback is that the return damage gives mana, and if the Blade Mail carrier isn’t tanky enough, it might not be worth it. The armor bonus is not huge, but still significant.

Maelstrom: “25% chance on attack to cast chain lightning for 150 damage to 4 targets. After every jump, damage is reduced by 20%.”
Great AoE DPS on heroes with good attack speed. The best Maelstrom carriers by far are Hunters because of their double attack and high attack speed in general. Medusa deserves a special mention - her Split Shot pretty much guarantees a lightning proc with every attack.

Skull Basher: “25 Attack Damage. 25% chance to stun targets for 1.5s.”
A bit more valuable than Maelstrom on melee heroes (especially Assassins), because the disable prevents the target from fighting back and the single-target proc is more valuable when you are trying to nuke targets down 

Octarine Essence: “Reduces ability cooldown by 30%”
Great on heroes who can realistically cast their spells more often with lower CD. Octarine Essence is great on heroes who survive for longer and have spells that they can cast multiple times in a fight – Bristleback, Necro, QoP, Doom, Abaddon, Omni.

Great in Hutner strategies. Makes reaching the full (6) Hunters bonus much easier, which is a big benefit because the difference between the (3) and (6) bonuses is significant. Extremely powerful on high DPS non-hunters as Shadow Fiend, Troll Warlord, Sven, etc.

Obviously quite good on heroes who are likely to accumulate kills. Moreover, the attack speed bonus is great in itself. How good it is in practice is strongly related to how early in the game you get it. The earlier, the more gold you'll be able to get from it.

More versatile than most other Hats because it's useful even if you're not running a Demon strategy. It can serve as a damage-increase item on any normal carry with an active ability (e.g. DK, Mirana, Wind, etc.).

Decent item on hard-hitting three-star melee carries like Lifestealer, Beastmaster, Lycan, CK. Not very versatile.

Obviously S-tier against Mage lineups (i.e. S-tier in a Mages meta) and lower-value against lineups that deal less magic damage (e.g. Hunters). That said, in the late game most lineups deal at least some kind of magic AoE damage so it’s never truly useless. One problem it has is that it forces you to bunch-up your heroes to get more value out of the item, which is not great a lot of the time in the super late game when you're trying to avoid powerful AoE effects.

Like most other hats, the Fedora could be used to reach the maximum (4) Brute bonus without actually having four Brutes in your lineup. It's not as strong as the Hunters Hat (Crown of Antlers), but it still could be useful for most lineups with Brutes.

Mango Tree: "At the start of combat and every 2.0 seconds afterward, the Mango Tree throws a mango up to 2 cells away. Any unit that touches a mango eats it to gain 50 mana and 100 health."
Decent in Warlick and Healer lineups, but the total heal/mana gain isn't something game-breaking, which makes it a relatively low-impact item.

Eul's Scepter: "+5 Mana per second. Whenever a unit attacks the equipped hero, it gets cycloned into the air, becoming invulnerable for 2.5 seconds."
Decent on backline squishy heroes who don't like getting focused and who can use the faster mana generation (Keeper, Shadow Demon, possibly SF, etc.).  That said, quite underwhelming for the item tier it's in.

Moon Shard: “+80 Attack Speed.”
The best DPS item in the game for a lot of heroes. It’s the improved version of Gloves of Haste – as we mentioned, heroes get higher damage with higher star upgrades, but they don’t get higher attack speed. This means that on average, bonus attack speed increases a high-star-level hero’s damage a more than bonus damage. Because of this, Moon Shard is one of the best items you can put on carries like Dragon Knight/Terrorblade/Slark/Luna.

Refresher Orb: “After casting an ability, refresh all cooldowns and restore 50 mana to equipped hero. Additionally reduce remaining cooldown time by 50% and restore 25 mana to allies 1 cell away. (once per battle)”
Similarly to Bracers, a great item on frontline crowd-control units like Tide/Disruptor. Refresher also benefits heroes in the proximity of the caster and doesn’t require the caster to fall to low HP, so it’s much better than Bracer on backline units with important spells like Lich/KotL/etc.

Butterfly: " +35 Attack Damage. +35 Attack Speed. +35% Evasion."
A great mix of DPS and survivability for any late game carry hero. Best used on frontline carries like Terrorblade, Lycan, etc.

Vladmir's Offering: "Allies within 2 cells get +8% damage, +5 armor and +10% lifesteal."
Due to the quite good area of effect, it provides amazing utility to a big chunk of your team - damage, survivability, and sustain.

Black King Bar: “+45 Attack Damage. Equipped hero becomes Magic Immune for 10s once the first enemy has 100% mana.”
(What the discription means is that the hero doesn't take magic damage, but is also immune to crowd control from magic sources.) BKB is theoretically an extremely powerful item for carries because in the late game every comp has some kind of control. That said, in Underlords each hero has only one item slot, which means BKB clashes for the slot with other great DPS items (Madness/Moonshard/Butterfly/etc.). If you have BKB but you pick another damage item afterward, you can choose to put BKB on one of your important frontline casters to ensure that they don’t get disabled and killed before they cast their ultimate.

A better version of Eul's. It’s best used on the frontline in an attempt to counter the other player’s important control heroes like Disruptor/Kunkka/etc. It can also be used on the backline to counter Assassins. The mana regen is good to allow your units to get off their spells first. That said, the disable is pretty unreliable and requires active positioning management in the late game to make good use of it.

Neither the HP nor the debuff is amazing, but together they add-up to provide good value. S-tier on Medusa because she applies the debuff on multiple enemies and can use the extra HP. Otherwise, it’s best used on heroes who could use well some extra survivability (e.g. one-star Ace units, or carries who can use the extra HP like Lycan or Slark).

The key here is "instead of their normal damage". This item shouldn't be equipped on your main carries, but it could be used on secondary damage dealers with good attack speed - utility Trolls (like Witch Doctor), low star-level Hunters (two-star Drow or Wind, etc.). Against tanky lineups the DPS of these heroes will increase significantly (ofc. most notably against Brawny).

The item is a bit hard to utilize – if you put it on a frontline tank, it will trigger before most of your units are damaged and it will provide very little value. Because of this, Mekansm is best used on a 2nd line unit as a counter to AoE damage - A-tier versus lineups with a lot of AoE (mages), and low-value versus lineups that focus-fire (Assassins, Hunters). It’s possible to use it on a frontline unit if you have a large frontline that spreads the damage it receives more equally – Warriors and Knights).

The burst damage is great in lineups that want to burst down enemies anyway (Assassins, Mages, even Hunters). It’s not that good in lineups that love prolonged fights like Warriors/Knights/Warlocks. There you’re much better off picking a normal DPS item. That said, it’s worth mentioning that Dagon never clashes for item slots with other items because it’s irrelevant on which unit you put it.

Not bad in theory if you mange the positioning closely and make sure the Diffusal hero hits an enemy with an important spell. That said, this is very hard to achieve and the raw stats are quite underwhelming.

Bloodthorn: “+70 Attack Damage. When an enemy hero has more than 75% mana, silence them for 5s. Attacks against the silenced hero can't miss and crit for 140%.”
DPS-wise it’s fine, but the main selling point is the silence. It will usually help you silence and nuke down one of your opponent’s main tanks/crowd control units: besides silencing, the debuff makes it so that every ally hitting the target crits for 140% DMG. The 10 sec CD means that if the item is on a backline damage dealer, it’s very likely that it will activate at least twice in a fight.

Divine Rapier: “+330 Attack Damage. If Divine Rapier is in combat and you lose the round to another player, they get the Divine Rapier.”
Arguably the best item in the game if you are one round away from losing. It gives the highest DPS increase out of all DPS items. The risk of giving Rapier to your opponent is meaningless if you are about to lose anyway, but it’s something important to consider in a more highly-contested game. If you’re doing well, you usually want to skip the item.

Best used on a very tanky frontline hero who will survive and burn enemies for long (Axe, Pudge, etc.). The main benefit, however, is the miss chance, which reduces the DPS output of the enemy team.

One of the best items in the game versus physical damage comps because of the AoE attack speed slow which effectively reduces enemy physical DPS a great deal for the 4s duration. That said, the damage from the nuke helps enemies charge a bit of mana, so it could be a bit counter-productive against comps that kill you with spells rather than attacks. If you want Shivas to hit the enemy backline instead of frontline, you can put it on an Assassin if you have one in your comp - this is extremely useful versus Hunters. Versus Assassins, you can put it on your backline to make sure it hits the enemy main carries.

Heart of Tarrasque: “+2000 Health. Regenerate 2% of Max Health every 1s.”
Pretty much the best tankiness item in the game. Its drawback is that it gives nothing besides tankiness. Because of this, it’s most useful on heroes that are not only frontline tanks but also deal decent DPS like e.g. Lycan, Doom, CK, possibly Slardar. It's absolutely great on inventors because of the higher explosion damage.

The zombies are pretty powerful, but the very high HP and armor are just as good. An additional tank with 4k HP on the frontline shouldn’t be underestimated. The big drawback is that the AoE is very low and it's a bit hard to fully utilize it in a lot of lineups, especially in the super-late game when you want to spread-out.

Horn of the Alpha: "2.0 seconds after the battle begins, summon a powerful Thunderhide Companion."
Best when your main carries already have items, becaue it gives you an additional poweful unit and it doesn matter who on your army holds the Horn. That said, it has a lower impact than most tier-five items.

Satanic: "+850 health. Whenever equipped hero is below 30% health, gain +200% lifesteal for 3.0 seconds."
Theoretically, it should give great survivability, but in practice, it has two big problems: first, it's not as reliable as e.g. Heart because if the unit doesn't hit during the 3s duration the item is almost useless. Second, it is most useful on carries, and it provides zero DPS, which is a problem in most cases.

If your lineup is clumped-up, it's a great item to put on a hero in the middle of the second row. A frontline carry can make great use of 6 extra seconds of life. That said, the item is still a bit unrealiable and forces you to clump-up your heroes in the late game.

Quite hard to utilize in practice. Often the hero resurrects after the team is dead and gets killed again by the surviving enemy heroes. Best used on frontline utility heroes because the resurrection resets the ult CD and because they are more likely to die at the beginning of the fight.