Wrath of the Lich King by far. I wish they would've implented Azjol-Nerub as either a very large raid or questing zone, soomething even Blizzard finds it a pity that they haven't. I would've liked more Arthas and Sylvanas/Jaina interactions, for the rest it was a great expansion. Still easily the best.
Good thing Cataclysm has so few votes. Funny thing is trough, that during the Cata launch, Cata was winning the same poll by a landslide. Now WotLK is :P
WotLK > TBC/Vanilla (equally as awesome, slightly prefer TBC) > MoP >>>>>>>>>>>>>>>>> Cata