I feel a bit daft saying this, but I loved the OST to Modern Warfare 2. The music in that game always keeps me going back to the singleplayer despite having done it about 8 times. Especially the music for the end sequence (boss fight, not credits).
http://www.youtube.com/watch?v=HChdE0putu4
Cue emotional music at 2:11
http://www.youtube.com/watch?v=HChdE0putu4
Cue emotional music at 2:11