For me, the greatest game soundtrack has to be that of my all-time favourite game: Grim Fandango. The game itself combined film noire with Mexican folklore (in addition to Aztek and other things). Likewise, the game's OST consists of Swing and Mexican music. A link to the soundtrack is below (a version of the remastered version can be found elsewhere online as that game was recently remastered after 16/17 years).
http://www.grimfandango.net/downloads/soundtrack
Additionally, it's definitely worth checking out other work of Peter McConnell as he is a musical genius and very dynamic with his game compositions.
To pick a single song from the OST, here is the first track - 'Casino Calavera' (the remastered edition):