I am aware that you should not trifle with the label "Best game I've ever played." When I talk in the office with colleagues through PC games, then comes back and back to the topic of favorite game. Considering nevertheless of games like Half Life 2, Skyrim, Diablo 3, Portal 2, and so I say that Dead Space 2 PC game is the best that I've ever played.
Maybe it is because I have seen as a child secretly Alien 2, and I'm so "biased" accordingly. Anyway, the atmosphere of Dead Space 2 is unbeatable: insulation, space and horror; Light and shadow; Soundscape -> everything perfect! What more do you need?