Hardest attempted: Mile High Club from Modern Warfare - 20 some tries in, I decided it wasn't really worth it anymore - the controller was aimed at the TV, after all.
Hardest succeeded: At the moment, I can only think of Complete MW2 on Veteran, although I keep hearing that it was much easier this time around due to no infinite enemy respawns and less grenade deaths - though I'll tell you taht the firefight on the White House lawn was harrowing...
Most tedious/time-taxing: Zombie Genocider from Dead Rising. But ~53,300 corpses later, you get THE REAL MEGA BLASTER!!! Psychos are less of a problem now.