Insane or Hero of Shattrath. Not hard, or even difficult, but mentally draining. I can barely do any grinding achieves now.
Hardest skill wise was probably "Glory of the Icecrown Raider (25 Player)" while it was current. I don't even remember what most of the achieves were, just that the Sindragosa and LK achieves were bitches that took lots of attempts.
I think some of the other Glory of the ____ Raider achieves were harder, but of the ones I finished as current content, the Icecrown one was top.