The most powerful abilities (especially with swaps):
Beast: Fjord Worg pup, Tito, Moon Moon, Frostwolf Pup and various foxes (e.g. Alpine Foxling)
Dragonkin: Chrominus
Elemental : Core Hound Pup
Humanoid : Lil’ Bad Wolf
Undead : Stitched pup
Undead: Crawling Claw, Creepy Crate, Landro’s Lich, Unborn Val’kyr, Lost of Lordaeron, Lil’ KT
Magic: Gusting Grimoire, Lesser Voidcaller, Lofty Libram
Humanoid: Sister of Temptation
Elemental: Droplet of Y’Shaarj
Flying: Brilliant Bloodfeather
Undead : Son of Sethe
Elemental: Ruby Droplet
Great when used against Legendaries or those few trainer pets that need over 500 damage to hit (e.g. Stitches Jr.)
Dragonkin: Nether Faerie, Sprite Darter, Yu’la
Magical: Ethereal Soultrader, Mini Mindslayer
Flying : Many, including Brilliant Bloodfeather and Jade Crane Chick
Critter : Darkmoon Hatchling and Lovebird Hatchling
Humanoid : Qiraji Guardling
Magic : Jade Owl
Undead : Vampiric Batling