Scariest game???? It's Amnesia: The Dark Descent by a long way, the scariest PC game ever made. I also voted for System Shock 2. System Shock 2 is a classic in it's own right and gave me many sleepless nights.
True scary and survival games are actually kind of rare. I mean almost all games focus on action and leaves the rest to the scary experience. But this one is not. So it fits perfectly as a SCARRRRYYY GAME...
About Amesia, this one is an unforgettable survival horror experience. Starting the game with no memories from the past and experiencing Daniels tormented memories and regaining them bit by bit, wow...i just loved it! The storyline was amazing and was one of the strongest points of the game! Environmental interaction was top notch, never before experimented. Amnesia makes full use of the environment, and this manipulation of objects, makes us really feel as if we are in the game playing as Daniel. This just increases the scary quotient to dizzying heights. Another one that I loved was the puzzles. We are in this holy c*rap situation trying to understand how we came here, and the hell to get out of here, and we actually need to use our wits to be successful. That's how it happens in real life.
Also one that makes the game really scary is that we don't have any true weapons! No guns nothing! ***! We actually have to run and find hiding places, or use rocks, chairs and other obstacles to stop these monsters. But this is just only for the time being to just give us more time. These monsters are just as intelligent as we are, and they tear through these obstacles and the only thing we have left to do is to hide in the dark. But here's where "Sanity" comes into place. By staying in the dark, staring at monsters, we will be terrorized by visual and auditory hallucinations. So this makes use of the light and shadows very well, and doesn't even give us the chance to stay hidden or do nothing for a long.
Now I believe everyone understands why this one is a really really really SCARRRYYYY game. Play this game to understand the "true" definition of scary.