I would say.. Resident Evil 1 & 2, Doom games, One Unit Whole Blood, Aliens games, Silent Hill, Splatter House (arcade), Friday 13th (NES)

You probably wouldn't consider it to be a horror game but I highly recommend the Metro trilogy. They're claustrophobic (at least the first two), superbly atmospheric and full of countless terrifying dangers. Probably the creepiest part of all of it is that it's a lot more of a realistic take on a theoretical post-nuclear wasteland than, say, Fallout. Everything you experience in the Metro series is, at least, plausible. Even in Exodus, the most open game in the series, you always feel immersed and you feel as though it's entirely possible that these terrifying dangers are what you might encounter if you were unfortunate enough to be stuck in a nightmare world like this.

And that's without mentioning the human danger. Whether you're facing a hostile faction basing itself off of the Nazis or the Communists, or you're going against feral humans who've gone mad due to any combination of radiation, isolation and bad air, all of the human opponents you face also feel terrifying.

Worse still is when you have to deal with both human and mutant opponents at the same time. Then, it comes closest to horror territory.

So yes, to me, the Metro series is the pinnacle of horror gaming even though not everyone would consider it a horror series. To me, I find atmospheric and psychologically-tense fiction to be a lot more terrifying than traditional horror.

Clive Barker's Undying scared the %@k out of me when I was 10 years old and relatively new to pc games. Good memories.

I would recommend the way underrated game 'The Swapper' which has a very nice spooky atmosphere.

Shadow of the Comet, good old classic game
One of the few games which reflect Lovecraft's horror in its spine-chilling purity

I would say Silent Hill 1 and 2, for the chilling imagery that is still ingrained in my mind, and all the hidden meanings behind its characters and situations.

It was already recommended more than once, but it can't be repeated enough: play the Silent Hill games.

Here's one that I doubt many people would even consider a horror game: Minecraft (Survival Mode).

Wait, don't downvote yet; hear me out!

Imagine being placed in a world, alone, with nothing to help you survive other than your smarts and the clothes on your back. At night, strange creatures roam the woods and deserts, some of whom literally go out with a BANG!! That crude hut made of soil and logs is constantly destroyed night after night until, at long last, you build a torch and a door to seal your home from the outside world.

Time passes. You have expanded and reinforced your home, even discovering the art of building with obsidian. One day, as you are lining your door with obsidian, you accidentally drop the bucket of water you had meant to use on your indoor crops. The ring glows a hue of purple and pink, shimmering like a pool of water slightly stained with blood.

That should have been the first warning - congratulations, you've just opened a portal to Hell.

Minecraft is a horror game unlike most; it combines a spooky, child-friendly horror with the dread that comes from isolation, the knowledge that you are truly alone. No one will save you, and not even the villagers that you meet can speak to you in anything remotely coherent. If the crippling loneliness doesn't get to you first then maybe the spiders, zombies, creepers or the Endermen will.

Cryostasis: Sleep of Reason. I still hope it comes back one day to GOG.

Silent Hill: Shattered Memories

It is by far the greatest horror game I've ever played and the only one I find myself returning to frequently. Every time you play it your character and his path are a little different. Even NPCs will have different dialog with you. The atmosphere is tense, there's no combat and the creatures are terrifying. Excellent psychological horror and the best entry in the Silent Hill franchise.

Silent Hill 1-4 are all great games and are worth checking out.

Vampire: The Masquerade - Bloodlines.

Upon mention of this game, likely it will cause a reinstall. Not just is this one of the best RPG's of all time - and my personal favorite game of all time, BTW - but it's also some of the most twisted and horrific.

One of my favorite levels of all time, The Ocean House Hotel. Nothing is more spooky, scary, and twisted than that level - flying objects; ghosts; and twisted & macabre story of...why the place is run-down and haunted. Even after going through this game and level numerous times - it still gets me to jump and get scared, unlike most games. I won't go into too many details, as I don't want to spoil it for those who somehow have not played this masterpiece of a game...and especially this masterpiece of a level.

This isn't enough. I could go on about the twisted opening, as how you become a Vampire, after a night of....wild sex. How Two Twisted Twin Sisters run a nightclub...and how there's more than meets the eye to both of them and their club. I could go on about much more scary, twisted, and macabre stuff that goes on in this game - with say Pisha and the underbelly of certain movies over in Hollywood.

But, I'll just stop there. This game just needs to be experienced...and likely, its teeth will sink into you and your teeth will sink into it.

I recommend Prey, I am quite sure mimics in it qualify as Trick or Treaters too :p

Definitely worth playing twice with and without typhon powers

The scariest game I ever played was Corridor 7. But I was like five years old. Anyway I couldn't tolerate being watched by that big eyeball so I bailed on that game pretty quickly.

Bioshock