To me, music makes or breaks a game. The second most important thing, only after the story is the game's music and how it ingrains in your mind, getting you into the feel of said game.
Hitting a pretty broad genre here, but pretty much any of the soundtracks from Final Fantasy I can stick in and listen to for solid hours. And usually, when sticking in music for maps made with Feral Heart, you can bet I'm sneaking in some Final Fantasy here or there. I stray more towards when Uematsu was still on board, but even then, I like the job that Suzuki, Hamauzu and the others are doing currently with the franchise.
Zanarkand will always be a the sole favorite from that series though.
So, what game soundtracks are your favorite? Only got a certain song? The entire soundtrack? An entire franchise?