Vanilla: Can't really say i was new to the game in 06 and didnt get to play with the high end content. but id say the world itself and the races
BC: Blood Elves and there tragedy i rolled a be paladin since they were the badasses of the light they took what they wanted from the light and still served up revenge to those who wronged them.
Wrath: Death Knights,Northrend and in general the lore of the expansion it was top notch questing was awesome.
Cataclysm:Revisiting old zones and seeing how they have progressed.Goblin and Worgen were pretty good
Mist of Pandaria: Pandaren Monks and Pandaria in general pretty much the expansion ive waited for for quite some time.