Since C'Thun preFix wasn't killed, and Ragnaros only was a GearCheck, i vote for Vaelastrasz...
We had BWL on farm, everyboss went one shot(even Nefarian with Black/Red), but Vael wiped us 2-3times at least per week. His bombs allways hitted your healer first, so after 2 or 3 bombs, it wasn't healable anymore... Costed us a lot of motivation and gold :P
Also Horseman at lvl60, was sickest.... The timing needed, and the proper rotation to find wasn't easy but after our firstkill it went quite well....