Well... I think the Bowser stage(s) in Mario Galaxy. I mean, they were pretty easy (Dark Matter Galaxy, though...) and I've played/seen others play the final level literally about ten times (once to unlock the Purple Coin challenges, once again to unlock Luigi, twice more to repeat those last two for Luigi, all those previously listed again when I saw my brother do it, and two or three more times between the two of us just for kicks), but they're still totally epic.