Amnesia: The Dark Descent, a first person survival horror. A game about immersion, discovery and living through a nightmare. An experience that will chill you to the core.
An incredible demonstration of the medium's capacity to deliver art intertwined seamlessly with scares and entertainment. A masterpiece of atmosphere slowly building suspense. Make sure to play not to win but to be immersed in the environment, as helpfully included in the instructions. The water monster is terrifying. Maybe a bit too short and the puzzles feel a bit at odds with much of the style of the game, but these are only superficial gripes with one of the scariest and most unique gaming experiences I've come across.