A lot of games can fit into the horror category without being a horror game (if you get what I'm saying). Personally, I like Amnesia: The Dark Descent because it makes you have to scavenge around rooms for parts, and if you don't, then you get an increase in insanity. Another favorite horror game is Outlast because the most terrifying moments come from your thought process. Some other horror games that are right below my favorites are Dead Space, Five Nights at Freddy's, and undertale (if you count the chilling scenes of seeing your saves getting deleted terrifying)