Hardest game I've ever beaten would probably be Ninja Gaiden on the NES.

Hardest game I've ever played? That's a tough one...the only FPS I've ever beaten was Goldeneye on the N64. I tried playing Doom but it was too hard for me.

The difficulty of a game depends on the player who is playing it. Some people are good at FPS's, some are better at a standard action game like Mario. I've noticed an increasing trend towards violent, gritty FPS's as time has worn on, and it really puts me off my soup, because I dislike a) graphic violence and b) FPS control schemes.