The hardest I've played is probably two days straight (no sleep) of 4-player Goldeneye on N64. Oh, you meant difficulty? TMNT on NES was impossible. Well, almost any NES game was.
IWBTG? Hard? Hahaha, only on your first playthrough. It gets easier really fast. The hardest game I've played is World of Goo. I don't know why it's so hard for me.