I think MoP is really the most fun. Tons of fun stuff to do.
Vanilla - was neat and new. Questing was pretty horrible though. Running all over the place was pretty lame imho.
BC - pretty friggin awesome. Putting groups together for heroic 5-mans was pretty horrible though. "LF 1 tank, then g2g" for hours on end.......bleh. Not very alt friendly either, which led to some level of boredom for me.
WotLK - was a good amount of fun. Being able to raid with more alts made this one a fun expansion. For me, Alts = raiding variability = don't get bored.
Cata - the remade 1-60 was phenomenal. Made leveling fun again (and quests were significantly more immersive than before). Getting randomly burned up by DW was pretty funny too. Even easier to raid with alts - which made this a really fun expansion for me.
MoP - the sheer amount of content is just insane. I feel like I'm always doing something different, which is a ton of fun for me. imho - questing is the best its ever been. Kudos to Blizz so far on this one. It's still early tho....so tough to rate the expansion as a whole.