In terms of atmosphere, story, how engrossed you were with the game, the survival aspects e.t.c

I am intrigued, personally i think the misery mod for S.T.A.L.K.E.R. CoP is terrifying, the atmosphere kills and the limited supplies make it intense. Its not classed as a complete horror game, but its monsters at night are invisible until they are like 2 meters from you.

Not to mention the blowout which occurs most days, basically a radiation storm that if you dont find cover within say 2 minutes your dead and have to start again. Its insane.

Outlast is also scary, but i think that stalker beats it on every level.