The first game to legitimately scare me. I went in completely blind to beat the game in one sitting in an overnight play session in complete darkness, with good headphones.
My only stumbling point was early on, I incorrectly thought the way to advance was to stack things to climb higher in a sort of rudimentary physics puzzle (that's never a solution in the game) when I was supposed to just push a button that was pretty much in plain sight.
You can cheese your way out of any scariness by ignoring the game mechanics (looking at certain things reduces your "sanity", but looking closely at the scary stuff takes away a lot of the fear of the thing), but if you go into it with the intent to play it straight, it's a fantastic game.