In the comments, go.
Mine is a draw between two. I simply can't choose one because each of them fit their respective games perfectly. I love every MotorStorm song, but if I had to narrow it down then these are my 2 number ones:
Monument Valley: Hyper - Hot Rockin'
Pacific Rift: Hadouken! - Liquid Lives (Noisia Instrumental Remix)
So what's your favourite?