I have to expect this from this forum... =P
The hardest game I've played isn't a Mario game. =P
Devil May Cry 3: Special Edition.
I've never played any other Devil May Cry and I might buy the 4th, way in the future.
DMC is not easy at all. After a lot of playing it, I became pretty good at it, but seriously, the first time you play the game and for a pretty long while it's not easy.
I mean I fricken UNLOCKED Easy mode. I UNLOCKED it. In a game where you have to unlock easy mode, you know it's not going to be easy.
It's still amazingly fun and I've become quite good at it.