15. Half-Life + HL: Source & Black Mesa mod (PC also on PS2)
I think this game really needs to be put into perspective to truly appreciate the revolution it brough to the FPS genre and general single-player storytelling in video games. It came out in a time where FPS' were all arcade shooters like Quake, Doom or Unreal, had little to no narrative, poor AI, unrealistic damage models (didn't matter where you hit someone, the damage was the same) and where everyone was a beefed up generic soldier (actually, not much has changed in that respect). Half-Life broke the mold. You played a geeky scientist, headshots would actually kill, the AI actually flanked you or flushed you out with grenades (rather than running into walls like a lot of other FPS' of the time), the narrative was excellent and was immersively told throughout the game world.
Quite simply, it was the first game to truly combine all these aspects into one revolutionary package. The narrative was incredible for the time; for the first level you didn't even get a gun as it just built-up tension and set the scene for the rest of the game. The lack of cut-scenes and the fact that everything was seen in first-person also meant that a greater level of immersion could be maintained making you actually feel like you were Gordon Freeman. It also balanced the action with superb level design and puzzle elements requiring you to actually use a modicum of thought as opposed to the pure gibb-fest of previous FPS titles.
Having said all that, if it didn't deliver in terms of action gameplay it wouldn't have been so highly regarded. It delivered! Surface Tension is probably still one of the best single player FPS levels I've had the joy of playing through with some of the best FPS action gameplay and level design seen in a game.
It's also worth remembering that the mods that sprung from this game (namely Counterstrike, but others like Day of Defeat as well) have gone on to become some of the greatest and most popular multiplayer FPS' in the world, many still being played today. Basically, nearly every FPS on the market today has been influenced in one way or another by Half-Life.
