Nier has the best soundtrack I've heard in anything. I didn't think it qualified but if it does, by the gods, it's amazing. It has a track called Gods Bound By Rules. That's all I'll say.
I'm surprised someone else knows about Alien Shooter. It's a fun game.
I have another game or two to offer, though. The first is The House In Fata Morgana. It's the best visual novel no one's ever played, and especially the soundtrack deserves more praise.
It can be a bop...
It can be heartwarming...
It can be moody...
It's just a beautiful, wonderful, harrowing, gripping, maddening, unforgettable game, and more people need to play it. I'd post more soundtracks, but discovering it as you go along is the best thing about it.
The second is Umineko. Umineko's excellence can't be fit into one post, mostly because there's more than 200 tracks in the game by my count.
But have one excellent song anyway: