WotLK. I have played just in WotLK, cata and MoP and WotLK was by far the best, then MoP which is slightly better than cata. In WotLK the game was new for me, had good raids and instances imo, a lot of miscellaneous stuff to do and brilliant PvP compared to cata and MoP at least.
Cata was just boring, had little content, bad to mediocre raids and horrible PvP, this made take a break a few times, the only time I took a break during WotLK was when the pre-cata shit occurred, and imo the game is always at it's worst during the pre-expansion stuff.
MoP is just slightly better than Cata, it has made me take a break already and the PvP is even worse now than what it was during cata. However, there is some other content which gives me something to do now and then so it keeps me playing as I lack other games to play. I give MoP a give minus for the asian theme as well, I just don't like it at all.