Silent hill 1 or Resident Evil 1 for me.
Silent hill for the fact that that game was truly F**ked up. The rusty blood colours and the air raid siren? Once you knew what that thing meant you were like "oh crap...It's getting bad again" and the constant darkness? I mean, not like RE darkness where there was lights and stuff...just pratically constant pitch black and the static on the radio? You Heard it was there before you saw it? those were some pretty messed up moments.
Resident evil because you were given literally *NO* ammo and you had to wade through all this mess and back out again. It was madness and it was really the first survival horror game around. Scared the whits out of me. Oh and:
(SPOILER)When in RE 1 you see the Tyrant in his tube and barry messes around with his controls then it breaks out and floors barry then *you* have to fight it...By yourself!?!? That was stupidly scary!