Had to go with Cata even though at the start it was my favourite expansion by a long chalk.
T11 was my favourite raid tier ever.
I LOVED the old world revamp, some of it is the best questing in MMO history imo.
But there was a few things that I did not like.
5 mans:-
4.1 run the same two 5 mans over and over again till you vomit.
4.3 run the same three 5 mans over and over again till you vomit.
No other real choice of earning Valor outside of that.
T12 and T13 I really did not like firelands or DS very much.
and 10 months with no content is way way way to long.
I am really enjoying MoP just now and am really looking forward to 5.2 (loving the rate of content release).
I really hope it does not go the same way of Cata though.