Heart of Fear.
But yeah, MoP is my second favourite expansion along with TBC. Wrath is very close behind (or equal), though.
Heart of Fear had some extremely cool bosses. I would say Blade Lord and Wind Lord were duds (Blade lord fighting the wind thing was a neat idea but too late in the fight and too trivial). The rest I liked, and Vizier is legitimately one of the top bosses of the entire game's history.