Mists of Pandaria
Cataclysm for me. Hopefully Warlords of Draenor or Dark Below or w/e is better than both of these xpacs though.
Firelands was really good, but that's kind of it for cata, sadly.
MoP had 3 really good tier imo.
FatSharkYes - Kazzak EU (http://www.fatsharkyes.eu/)
MY DK - "Stand in the fire, DPS higher!"