Hardest game I've completed was Ninja Gaiden 1/2 on Master Mode. After beating that insanity. FPS games on their hardest modes are cake walks.
Heaven or Hell mode for DMC 3, it almost broke me a couple of times.
nah heaven or hell is one of the easier difficulties cause even though you die in one hit the same applies to your enemies. the real challenge in that game is dante must die (or vergil must die) and ninja gaiden 2 master ninja is more artificial difficulty due to non stop projectile (incediary shuriken) spam rather than real difficulty which NG1 master ninja was mostly real difficulty
I like when there is an almost impossible difficulty. It's there just because, why not right? And if you're talking about the boss I'm thinking of in Deus Ex, it was the easiest boss fight in the game. All you have to do is punch him while he is climbing over one of the walls.
"People die when they are killed." GT: A Lost Grenade