Bioshock, 1 1/2 years later, is still the best single player first person shooter experience on the market.
This game was built entirely for the single player, and in an era of multiplayer games and games built to straddle the divide, this game is purely for your solo immersion.
As you explore an art-deco underwater utopia-gone-wrong you encounter the remnants of the population: deranged, genetically damaged people called splicers who see all outsiders as threats. Along the way you pick up audio tapes left behind by scientists and luminaries of the failed underwater city of Rapture, and learn what befell it. Every area can be completed with a variety of approaches depending on how you want to configure your abilities that include sending electric shocks for your fingertips, telekinesis and fooling enemies into attacking each-other. The environment is highly useable and includes puddles of water that can be used to enhance electric shocks, security cameras, gun turrets, and other devices that you can turn to your advantage.
This game is totally immersive and if you turn out the lights, it is truly very creepy.