Best Horror/Halloween Game for PC

Not a horror game but the best literal Halloween games would be Costume Quest 1&2. =)

Sanitarium is a fantastic atmospheric horror adventure game from the late '90s. It's on Steam. The Dark Eye is another '90s classic based on the works of Edgar Allen Poe. Unfortunately that one isn't on Steam so you'll need to track down a physical copy. It shouldn't be expensive. Worth it if you like surreal horror IMO.

While I would absolutely suggest stuff like the Fatal Frame and Silent Hill series, I'm gonna keep my list limited just to stuff that can be found on Steam.

I have rated my list thusly:
Not Scary (NS) - Just pretending to be scary, or trying to be scary and not really making it. The goofy Halloween side of things.
Kinda Scary (KS) - Five year olds may not be able to handle it, but you can, easy.
Scary (S) - Okay, now we're talkin'. This'll keep you looking over your shoulder afterward.
Super Scary (SS) - Whatever hair you may have is gonna turn white. Pure horror.
Of course, your mileage may vary.

7 Days to Die (NS) - Single- or multiplayer survival/crafting/building game. Post-apocalyptic setting with zombies.
Alan Wake Franchise (KS) - Delve into an action-packed world where darkness is your enemy, light is your weapon, and fiction and reality collide.
Alien Breed Trilogy (KS) - Sci-fi shoot-em-up against ravaging alien monsters in the dark corridors of a mysterious spaceship.
Alien: Isolation (S) - Taking the Alien franchise back to its horror roots as you sneak, scavenge, and flamethrow your way around a killer xenomorph.
Amnesia: The Dark Descent (S) - Gothic horror with just a touch of Lovecraft sprinkled in, just enough to add spice without being overpowering.
Amnesia: A Machine for Pigs (KS) - Not quite as freaky as the other Amnesia, but I personally still rather enjoyed this steampunk horror tale.
Answer Knot (KS) - A short little freebie about checking increasingly disturbing messages on your answering machine.
Bendy and the Ink Machine (KS) - Kiddy cartoons gone terribly wrong.
Claire (S) - Aid Claire as she tries to find her comatose mother in the dark, twisted corridors of an otherworldly hospital.
Conarium (KS) - Playing this one right now myself! Pure Lovecraft mythos stuff, but still pretty good for all that.
Condemned: Criminal Origins (S) - You'll never look at store mannequins the same way again.
Costume Quest 1 & 2 (NS) - OMGoodness, so adorable! Mean ol' goblins are stealing all the candy, and you have to stop 'em with your costume-based powers!
Dead Rising (NS) - Haven't played the others yet, so I can't vouch for them, but the first DR is a heckuva lotta zombie-smashing fun.
Dead Space Collection (S) - Somewhat spooky sci-fi survival horror. I personally like DS3, but it is not included here because it's only on Origin and it skews more toward action than horror.
Distraint 1 & 2 (KS) - The unrelenting psychological horror of dispossessing people of their homes.
Distrust: Polar Survival (NS) - Guide survivors through a procedurally-generated abandoned Antarctic base that was attacked by a mysterious force.
Doki Doki Literature Club (SS) - A cute little visual novel about adorable girls who write poetry and make cupcakes.
DreadOut (S) - There are creepy Indonesian monsters lurking in the darkness, but it's okay. You've got your cellphone to protect you.
Duskers (KS) - You wouldn't think piloting a bunch of little drones around derelict spaceships would be unsettling, but here we are anyway.
F.E.A.R. Franchise (KS) - Action shooters with horror elements layered in.
Fausts Alptraum (NS) - Gothic horror puzzle and exploration game with wonderful art in the cutscenes. And it's free!
Fingerbones (SS) - My rating for this short little freebie may be a bit misleading, so I'll explain. I'm rating it SS not because it's particularly scary, but because it is pretty damn disturbing. Good tho'.
Greetings (SS) - This sci-fi horror freebie may be super-short, but it still manages to be the absolute scariest game on this list, hands down.
home (KS) - Make your own fate as you traverse the dark path home.
I Shall Remain (NS) - An action RPG cutting a swath of destruction through hordes of hideous undead. Way underrated, in my opinion.
INSIDE (NS) - A puzzle platformer starring a little boy being hunted down through a dark forest. Then shit gets weird.
Kraven Manor (KS) - Not a bad little exploration puzzler for a game made as a university project.
Layers of Fear (KS) - A painter trapped inside his own house, which has become a labyrinth of his own making.
Lifeless Planet (NS) - After crashlanding on an alien planet several light years from Earth, an astronaut finds he isn't the first human to set foot there.
LIMBO (NS) - A puzzle platformer starring a little boy being hunted down through a dark forest. Then shit gets- . . . wait, this seems familiar.
Lumber Island - That Special Place (S) - An island shrouded in the dark of night, an abandoned town along its shore, and you, trapped there all alone. Surely this will be just fine.
Murdered: Soul Suspect (NS) - You're a spooooky ghoooost! And also a police detective who has to solve his own murder using his new spectral powers.
Outlast (SS) - Haven't played the second one yet, but this one had me on the edge of my seat the entire time I was playing it. Tense, taut, and thrilling.
Oxenfree (KS) - Hanging out on the local tourist trap island after hours, drinking and smoking and just being teenagers about to hit adulthood. Then a portal opens and something comes through . . .
Paranormal (SS) - Don't let the fact that it's an abandoned early access game put you off. This really is no-foolin' one of the most terrifying games on this list.
Perception (KS) - While this game does have its unsettling moments, I'm really recommending this one more on the strength of its story and its interesting presentation and core mechanic.
Pony Island (NS) - Though I didn't much care for the backstory, the gameplay and quirky nature of this game more than makes up for it.
Realms of the Haunting (NS) - I was absolutely shocked at how good this game actually is. I even like the super-kitschy story and acting.

Ugh, okay, this has taken waaaaay longer than I had originally anticipated and I can barely keep my eyes open any longer. Time to get some sleep, then I'll add S-Z in the mornin'. Hopefully this is still a good enough start!

Hellblade, Outlast 1+ DLC and Outlast 2.

Since no one mentioned it (quiet weird). The Suffering It's pretty good and scary shooter game. Don't remember much about it, thought.

https://en.wikipedia.org/wiki/Alone_in_the_Dark_(1992_video_game)