Here are mine:
All are based on WarCraft 3's hostile units. Sorry if your units aren't based on them, but see this a future reference, if so.
Centaur
2 Ranged units, 3 Physical Units and One mage. Judging from this, you would need a mix of Health-based items, like HP Regen or Health Potions and some defensive items to block off their future ranged attacks.
Kobold
3 Melee units and one mage. Most have bash and one has slow. Judging from this, you need some sort of crowd control reduction item drops from them. All deal near-mediocre damage. So, no need for armor, but you might add some damage reduction items.
Bandits
4 melee units, two ranged. All have Shadow Meld, allowing to them invisible. So make their drops an invisibility detection item. Bandit Lord deals chaos damage, so you need a good armor item, or an item to reduce incoming damage. One has Envenomed Weapons, making it a lot hostile, so make their drops health potions, health regen/max health increase items. Also, give some mana potions.
Wildkins
All melee, one deals chaos damage and has bash + war stomp. You would need some health potions here and some crowd control reduction items here. Since they are melee, ranged units work better, so you would need some damage amplifying item for this. Mana potions/regen items would be good, since most of them are beefy.
Ancient Protectors
All of them are beefy and melee, but slow. Deals great attack damage and have high amounts of health. So, their drops would be mana regen/potions, since you would be spamming you abilities and some attack speed/attack damage items so you can take them down faster. Slowing items wouldn't be that effective, since theyre already slow.
Harpies
All ranged, and all squishy units, with one dealing chaos damage. Warcraft 3's Harpy Queen has Rejuvenation and Cyclone, making her a threat, so she could drop attack damage/spell damage items, or better yet critical chance/power items. Storm-hag fills an offensive supportive role, so you should make her drop crowd control reduction items, or better yet, an item that removes negative buffs. All other deal pierce damage, so you would need some attack damage/crit. items for them and some health potions/regen items.
Trolls and Orcs
Orcs are mostly melee and deal decent damage and have moderate to high health. You would need them to drop health potions/regen and some armor items. Mana potions/increase items wouldnt hurt too. Trolls on the other and deal moderate damage, a bit low health. So you would need attack damage/spell damage items to finish them off quickly.
Murlocs
Like the harpies, squishy, but they are a good variety of spells. Armor items and health regen/potions would do. Purging items/buff reduction items would be good.
Forest Trolls
All ranged, weak hp-wise. Their casters are supportive ones, which heal and support allies. So you would need an item which reduces these buffs or an item which has purge. All the rest are pierce-damage, meaning they deal more damage to melee units, and Warlord has Trueshot on him, so you would need a items to boost your armor/health or deal more damage. Add some health potions too.
Shaman
Supportive units. You would need attack damage items to take them down or items which cleanse you of negative effects and a purging items for the positive effects they bring, especially Bloodlust and Lightning Shield.
Tauren
Extremely beefy, hp-wise and deals good attack damage, but are slow. Pulverize is basically a cleaving attack, so you would need items that increase your health, like health potions/regen items. Also, attack damage and attack speed items in order to take them down faster. Crit items could do.
