Vanilla - Leveling
TBC - Insanely overly extreme attunements.
Wrath - Ulduar. What a crap raid.
Cata - Redone 1-60 quests.
MoP - The art, the raids, the dungeons, the pvp, the Pandaren, the legendary cloak, pet battles, challenge mode gear.
Everything in MoP was bad. Everything.