Mists of Pandaria
it at least attempted to raise the bar above what wrath babies had become accustomed to.
There is no bad RNG only bad L2P
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.
Envy - Kazzak EU (www.envy.si)
MY DK - "Stand in the fire, DPS higher!"