Wrath was the best. It expanded the game to a point where it had the highest number of subs ever, one of the greatest raids ever (ulduar. basing that opinion off of the lore of the raid, the fact that it's the first to put in "heroics" into raids, and it was just actually FUN), and expanded lore by a good chunk. It also related back to warcraft III, which many people have played and enjoyed.
I joined in TBC and I've liked wrath more than anything. But I also have nostalgia for it, so my opinions are bias a bit.