Mine is Silent Hill 3. Its subject matter revolves around the fears of a teenage girl, like death of a parent, puberty, unwanted pregnancy (with a demon fetus, no less), mirrors (THAT ROOM shivers), and stalkers. The atmosphere is so oppressive and I cant play the game without having something else playing in the background, its that horrifying. I managed to finish Silent Hill 2 last year, which was like conquering my own fears. One day, I'll finish SH3. One day.
Outlast is 2spooky4me. Got pretty far in, but then just couldn’t get past the dread.
Never played it, but Alien: Isolation is a cool-looking game I’m not going to even bother attempting.