-Xaero from Quake3 on Nightmare skill. The game is 8 years old now and I still haven't beaten him.
Also from my chilhood days the robot sonic from that spaceship level in Sonic 2 was a P.I.T.A for a young kid at the time ^_^.
Inspired by this thread I went back and managed to finish FarCry last night!
God that game was pretty big, not played Crysis (just the demo) but I can't imagine it is anywhere near as long as FarCry.
final boss in Metroid Prime on gamecube. Arghhhh!!!