If we go by absolute scariest, I would have to go with Amnesia: The Dark Descent. If we went by most scared at the time, I would say Resident Evil One on its gamecube release. Watched my brother play it when I was pretty young and it scared the daylights outta me.
When I was younger I played Vampire: The Masquerade – Bloodlines, scariest game I've played from memory. One of the earlier missions you had to go through a haunted mansion, took me like a month to get through it, considering I would play for a couple of minutes a day, get freaked out and turn it off.
When I first played Dead Space, that freaked me out a bit too, but I hired it from a video store, and I had a day left, and I had barely touched it because it freaked me out, and I just wanted to get the achievements, so I played through it, and now Dead Space and Dead Space 2, don't phase me one bit, and they're two of my favourite games.
When I was quite a bit younger the original Dino Crisis scared the crap outta me, gave me nightmares about dinosaurs for months, but I grew to thoroughly enjoy it. I'm not much of a person for horror games because of the suspense but Project Zero 3 (Stupid Australian title), aka Fatal Frame 3, was one of the scariest games I have ever played. Crawling around underneath a Japanese mansion trying to find the ghost of the characters dead husband when a crawling ghost thingy scurries towards you grabs you and shakes you with the creepiest thousand yard stare in the world...
They all scared the shit outa me I can't really place a degree which scared the most shit out of me. if it was a gatherer- and it's face looked like sumbody used a car buffer on it, i was scared 2 fuck.
Yeah it was late and I tend to be freaked out by zombie no matter the setting... I think the only "zombie" game that doesn't make me at least a little uncomfortable is Planets vs Zombies lol. The ship that you get Legion from in ME2 always makes me nervous too
Oblivion would get me jumpy whenever I went into Basements, because I knew at any moment a guard would come in, and my screen would spin around, and their poorly lit ugly face would be staring at me. Made me jump every time. Also, basement of the Anvil Lighthouse.