After much deliberation, I finally have a conclusion:

Outcast - PC

I find music is a very important aspect of games, responsible for communicating the tone of a scene to the personality of a character. Personally I find that a game with poor, or badly placed music can turn me off it almost immediately (thankfully the music in Stalker is optional).

I've picked Outcast for best soundtrack not just because of the quality of the music (composed by Lennie Moore and performed by the Moscow Symphony Orchestra) but because of how integral it is to the game. The feeling of being alone and confused on an alien world and all of the danger, discovery and enlightenment that follows is conveyed through the music as well as the gameplay. From the epic operatic chanting of the opening to the tribal stomping of the battle music it transported me into the world in a way that just could not have been done without the soundtrack.

Outcast at the time was one of the first games to have a full-orchestral soundtrack, which in today's climate is relatively common. However, I guess this is partially nostalgia, but I feel that few titles have managed to integrate the soundtrack and game so successfully. This was a tightly contested place for me (runners up included FF7, Doom, Mirror's Edge, Crusader: No Remorse and Radiant Historia) and to be honest, I think depending on my mood any one of them could have taken it. Still, every time I listen to Outcast I get returned to it's world in a way most other soundtracks do not.

Outcast - The Ancient Forest World