Here's a few of my not-quite-as-popular horror game suggestions.
Siren:
Want to know what most of the Silent Hill guys did later on? This was that game. "Sight-jacking" lets you see through the eyes of the monsters, and nothing creeps you out more than "seeing" them stalk you, drawing closer to where you're hiding...
Clive Barker's Undying:
While the Clive Barker name has been attached to a lot of junk games (looking at you "Jericho"), Undying is legitimately creepy, atmospheric, and severed screwed up in the best ways. It's a little dated, but well worth the asking price at GoG.
Haunting Ground/Demento:
Very similar to the original Clock Towers, this is basically ICO meets survival horror. It's eerie, often obtuse, and frequently frustrating... but definitely worth checking out for fans of the genre.
Cold Fear:
Everyone who played Resident Evil 4, and then was letdown by how unscary Resident Evil 5 was, should try this game out. It was a very obviously copy of RE4, but that's the point... it was a better RE4 experience than RE5 or RE6 were, with better atmosphere, monsters, and scares to go along with it. Really underrated.