Some game soundracks will never cease to amaze me, i have downloaded the ones i love the most and i'm listening them often more that other, regular music. Game music composers really did an outstanding job on creating the perfect melodies for the given games and making them complete that way.
My favorite game music/game soundtracks are from:
World of Warcraft-this game really has the most amazing melodies.. Lament Of The Highborne, Nightsong, Burning Steppes, Way Of The Monk songs are beautiful beyond words..
Dreamfall and Dreamfall Chapters with various beautiful and mostly sad songs..
Tomb Raider has awesome ambient music which suits the places Lara is visiting really well!
Vampire the Masquerade-Bloodlines-using some popular gothic metal songs and having a beautiful composed ones on its own..
Witcher with those superb slavic and folk melodies.
Silent Hill-music is beyond good and suits the game atmosphere so well..
and League of Legends has some good music too.