Fatal Frame. I first played it back in 8th grade when it came out. Could barely play it with the lights on, during the day, with all the windows on. And it had to be sunny. Game just got me for some reason.
Still to this day, (I'm almost 24), it's hard to play it with the lights off.
Amnesia is a respected 2nd.
Currently playing: Skyrim, Torchlight II, Half-Life 2, Jade Empire, the Last Remnant That's a dealbreaker.
Jurassic Park CD for SEGA CD. That game gave me so many nightmares as a kid. I hold that game as the scariest game I've ever played. No other game I have played in my 22 years of video game experience has given me nightmares as horrifying as the one I had for Jurassic Park CD.
Anyone who has listed any of the Resident Evils or any of the Silent Hills obviously have only played the most mainstream survival horrors. Every other lesser known survival horrors like Prenumbra, Clock Tower, Rule of Rose, Haunting Ground, Condemned, Phantasmogria all trumps REmake+Silent Hill 2 in every horror "criteria".
And Lol at dead space. It's focus on action defeats much of the horror aspect and even then it's mainly consists of Jump Scares which the other games did just as well.
The literally scariest game ever for me was Fatal Frame 2, the only game to ever truly give me nightmares and the one that fits every horror criteria perfectly and more than all the other survival horrors. Amnesia is 2nd for me but I'll admit I'm jaded after growing up since then and having already heard about it's horror making me prepared for it.
Zombies just can't be scary though. They're the most generic of enemies and not even strong ones.