As for bumping this thread up, here are my scariest games I've played:
1) F.E.A.R. (PC)
I actually didn't have the nerves to go past that next corner, on several occasions... Great atmosphere, mostly due to the lighting and the excellent use of sound and music (or better, the absence on many occasions).
2) Doom I and II (PC)
I had countless scares during gameplay. Especially those chilling imps with their scratching sound....
3) Personal nightmare (Amiga)
As mentioned before, this game has some gory imagery in there...