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.
Not a perfect game, but, even after 5 years, it is still the undisputed king of atmospheric horror and immersion. Great game to mod as well. If you have never played Amnesia, pick it up if you like: horror, fantastic story-telling, immersion, great interactive experience or love to throw **** around the room like a toddler with the upper-body strength of a bodybuilder.