Vote as you think wich 1 is the hardest:
Kalimdor:
Stone Cold Trixxy - Winterspring - level 19 pets - flying, dragonkin, beast
Counter: Magic, Humanoid, Mechanical
http://www.wowhead.com/npc=66466
Eastern Kingdoms:
Lydia Accoste - Deadwind Pass - level 19 pets - undead, undead, elemental
Counter: Critter, Aquatic
http://www.wowhead.com/npc=66522
Outland:
Bloodknight Antari - Shadowmoon Valley - level 25 pets - dragonkin, magic, elemental
Counter: Aquatic, Dragonkin, Humanoid
http://www.wowhead.com/npc=66557
Northrend:
Major Payne - Icecrown - level 25 pets - beast, mechanical, elemental (i hate that 1 the most)
Counter: Mechanical, Aquatic, Elemental (with defense)
http://www.wowhead.com/npc=66675#comments
Cataclysm:
Obalis - Uldum - level 25 pets - beast, flying, critter
Counter: beast, magic (i use dragonkin sindra), Mechanical
http://www.wowhead.com/npc=66824
Pandaria:
Hyuna of the Shrines - Jade Forest - level 25 pets - aquatic, beast, flying
Counter: flying, magic, mechanical
http://www.wowhead.com/npc=66730#comments
Farmer Nishi - Valley of the Four Winds - level 25 pets - beast, elemental, elemental
Counter: mechanical, aquatic (its faceroll)
http://www.wowhead.com/npc=66734#comments
Wastewalker Shu - Dread Wastes - level 25 pets - aquatic, beast, elemental
Counter: flying, mechanical, aquatic
http://www.wowhead.com/npc=66739#comments
Courageous Yon - Kun Lai Summit - level 25 pets - flying, critter, beast
Counter: magic, beast, mechanical
http://www.wowhead.com/npc=66738
Seeker Zusshi - Townlong Steppes - level 25 pets - elemental, critter, aquatic
Counter: aquatic, beast, flying
http://www.wowhead.com/npc=66918#comments
Mo'ruk - Krasarang Wilds - level 25 pets - flying, aquatic, beast
Counter: flying, magic, mechanical
http://www.wowhead.com/npc=66733#comments
Aki the Chosen - Vale of Eternal Blossoms - level 25 pets - critter, dragonkin, aquatic
Counter: beast, humanoid, flying (it can be soloed by http://www.wowhead.com/npc=68659 )
http://www.wowhead.com/npc=66741#comments
Elite 4
Thundering Pandaren Spirit - Kun-Lai Summit - level 25 pets - beast, critter, elemental
Counter: I am not telling you that
http://www.wowhead.com/npc=68465#comments
Whispering Pandaren Spirit - Jade Forest - level 25 pets - dragonkin, critter, elemental
Counter: 3x dragonkin pets
http://www.wowhead.com/npc=68464#comments
Burning Pandaren Spirit - Townlong Steppes - level 25 pets - dragonkin, flying, elemental
Counter: (tip snails)
http://www.wowhead.com/quest=32434
Flowing Pandaren Spirit - Dread Wastes - level 25 pets - aquatic, critter, elemental
Counter: Secret
Darkmoon Faire Tamer:
Jeremy Feasel - across from tonk challenge - level 25 pets - magic, mechanical, beast
Counter: I am not gona spoil you that
http://www.wowhead.com/npc=67370#comments
I merged some of the npcs in the poll (limit 15) by being smiliar hard or easy.