WoTLK will always be my favorite, because it focused on the Scourge and the Lich King. Burning Crusade drove me away from the game since it was about space aliens (Draenei and Burning Legion, yes, they're all aliens, not demons), and Cataclysm just had a dull villain (Derpwing and the Twilight Cultists). MoP... I would play, but I can't justify paying a monthly fee for such an old game.