What is the hardest level in your opinion?

Super Mario Galaxy 2 - Grandmaster Galaxy - The Perfect Run

Went in with 60+ lives, exited with a Game Over. Countless more Game Over's followed as I kept retrying. Since this was in my younger days, eventually I became so enraged that I threw my Wii Remote at the TV, breaking the screen. It has been a long time since I played Super Mario Galaxy 2, but I have never beaten that last level.

Banshee Broadwalk from Mario Kart 64. I kept messing up the zigzag bend on the course and kept coming below 4th. It took that many tries to get to Rainbow Road and it took 1 try to complete Rainbow Road.