For me, it was Super Mario Bros. The Lost Levels. That game was HARD. I couldn't get past Stage 8-3(the cloud level) until I got the gameboy version and used a gameshark to beat the game. Far Cry takes 2nd place because you can't just save when you like and the enemies like to sneak up on you, and X-wing for the PC is 3rd because I always get killed and when you use the invincible and infinite ammo options, it makes you play the level over again.