Callik

http://www.youtube.com/watch?v=HtvIYRrgZ04

For anyone who hasn't seen it.
Beats Leeroy imo

I never did any TBC raiding so can only go on classic raids. This is how my guild found things.

Easiest
MC: Gehennas/Golemagg
BWL: Broodlord Lashlayer/Drakes
AQ: Skeram/Fankriss
Naxx: Spider Wing/Razuvious

Hardest
MC: Majordomo/Ragnaros
BWL: Vaelestraz/Chromaggus
AQ: C'Thun/Twin Emps
Naxx: Construct Wing/Four Horsemen

Hardest of the lot? In terms of pure wiping we spent a good several weeks beating on Vael before we got him down.

Bout damn time.