Scariest game i've seen being played is Amnesia: TDD, hands down. Being somewhat afraid of the dark doesn't help one bit. Perfectly summed up here.
Lol'd. A very similar thing happened to me in Dark Souls, when I first encountered Kingseeker Frampt.
Granted he isn't hostile at all, he still almost made me run for the hills.
As for another scary game, Condemned 2 made me cack my pants on several occasions. Enemies actually being smart enough to be able to stalk you and ambush you out of nowhere can be rather... unsettling.
jk jk lolololol. Id think it was nightmare house. One of those games were i sit in the corner screaming at the tv, telling the evil bitch to stay away.
Amnesia dark descent scared me a lot, but not any more than fish's avatar does. my first horror game was also pretty scary. It was "road to the fiddler's green". I couldn't even shoot one zombie. So scared.