What is the scariest video game you have experienced to date, and what made it so terrifying?

Witcher 3, probably, I have never played anything close to a proper survival-horror game.

EDIT: Well, I actually have, and that's Hellblade, which, I'll be honest, it didn't do much on me, the only thing that was a bit unsettling was Valravn, that thing was really well made. Aside from that, though, I don't remember much of it, despite the fact I played it only 2 years ago, which goes to show the little impression the game left on me. 

Amnesia: The Dark Descent and Outlast. Absolutely terrifying.

I'm actually too scared to play Visage. Haunted house with realistic graphics and lots of scares is too much for me.

I have no problems with Outlast, Resident Evil, or Silent Hill. Even if RE7 scared me a bit.

I don't play many horror games, but some games have horror elements.

Nioh 2 has great atmosphere with some downright creepy moments. Plus it is a different style of horror, with the Japanese folklore background.

I don't play survival horror games but one visual novel (I think?) on the DS that really gave me the chills was 999: 9 Hours, 9 Persons, 9 Doors. It has sci-fi, suspense and horror themes in it with multiple bad endings. When I played it, I got first the "neutral" ending which was more or less a cliffhanger. Then in my second playthrough, I got the WORST ending which is pure horror. Never in my life had I been so scared with a videogame or a movie or anything, I was literally trembling and almost could not sleep that night.

Any David Cage Game. Writing that bad is frightening.

999 Spoilers:

Fear, it was the first game i played that made me jump out of my chair

Bloodborne. Something about the atmosphere an d the Cthulhu themes. I just wanted to finish that thing as soon as possible.