Maybe not whole soundtracks, but there are a handful of songs that I regularly go back to as musical therapy.
Skyrim - Dragonborn. I own the whole soundtrack, but that song, taken both in and out of context, is one of the most awesome pieces of music I've ever heard. Apparently, the men's chorus we hear chanting (then shouting) in dragon tongue was originally 30 actual voices, repeated, to sound like 90. This isn't really that big, but to me, it's the kind of thing that makes the difference between cool and cut-above-the-rest amazing.
Bastion - Build That Wall. I almost don't even need to elaborate. This is a song that reinforces the tone of the entire game. Hopeful, yet sorrowful; spiritual, yet very down-to-earth; simple at first glance, but there's a lot going on, when you look more closely.
Oh, and the voice of Zia is gooooorrgeous.
Bioshock Infinite - Will the Circle Be Unbroken (Choral Version). I'm not a spiritual man, but when I first heard this, combined with the inspired setting in which you hear this piece, well, I was very nearly made a believer, right then and there.
Bioshock Infinite - God Only Knows. If you can't tell just yet, I'm a sucker for a chorus. I believe the human voice is one of the the most potent things on this planet. A voice, lifted in song, is one of the most beautiful.
I was already thoroughly ready to call Infinite a prime candidate for game of the year... and then a barbershop quartet shows up. Well, bugger. It would have to be a pretty spectacular game to top Bioshock Infinite, or at least a better soundtrack.
Final Fantasy VI - The Opera Scene. I have about a half-dozen different voices echoing in my head, singing the lyrics over the 16-bit instrumentals. One of them is my own mother, a woman who can out-sing a 100-strong church congregation. Oh yeah. Right in the feels.
