at 80... with less than.. 10-15 people.. The Prophet Skeram is the hardest, the moment any caster gets MC'd the raid gets 1 shotted... lolz
(back to seriousness)
pre-TBC... well 1. obviously hogger.. but other than him.. hakkar with all the aspects up... probably toughest thing you could do from what i can think of...
TBC... Lady Vashj pre-nerf
WotLK... well i havent played any of the most recent stuff... but mimiron hard mode, pre-nerf.