That is a pretty difficult question, and you'd probably get a dozen different answers out of me depending on when you ask. Tomorrow, I'm probably gonna regret not picking some other game, but right now, I'm going with the soundtrack for Bastion.
I absolutely love weird, unique genre fusions, and Darren Korb delivered that in spades. It's been nearly ten years since Bastion's release, but I still remember how it contributed to my enthusiasm for the then-upcoming indie scene. With that kind of creative talent contributing, the future of gaming seemed pretty great.
From americana and strings, to heartfelt ballads, to sitar over hip hop beats backed up by some gnarly overdriven guitar, there isn't a track that I don't absolutely love. And despite its disparate influences, the whole thing just feels so strangely cohesive. It is totally unique, and stands up equally as well on its own as it does bringing to life the world of Bastion.