You don't play a hunter, do you?
My brief take on things:
Vanilla: Was what it was. Lots of promise, lots of rough edges.
BC: Strove to be something great, and there's definitely the most imagination and attention to detail here as far as environments go.
Wrath: Pinnacle of xpac-spanning storytelling in this game so far. Cohesive and very well done.
Cata: Again strove for greatness, but missed the mark... did a few things right, and a lot of things wrong.
MoP: Far better than Cata, but might be trying a bit too much to appeal to *everyone*
While none of these expansions were blemish-free, I'd call Wrath the winner, with BC a close second.