Before we get into this I should note, based on everything I've heard from multiple sources, Nier has the best OST of 2010. Having put all of thirty minutes into the title, I'm not prepared to endorse or condemn it - but I felt the need to at least note its absence.
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I'm not the most musically-inclined fellow, but as I looked at the list of contenders for 2010, there was really no getting around it - a good portion of games this year were truly treats for the ears. Some offer soundtracks the likes of which we've never heard, some break it down old-school to fantastic effect, and some are just there to deepen the atmosphere you find yourself drowning in. There are some real standouts...
The Frying Tengu by Anamanaguchi
Chiptunes band Anamanaguchi contributed the music to The Best Downloadable Game Based On A Movie Based On A Comic Book of 2010, and it perfectly captures the game's retro style and the book's classic-gamey sensibilities. The Scott Pilgrim game soundtrack is a high-powered homage to, and celebration of, the best video game music of yesteryear - and one of only two video game soundtracks this year that ended up on my MP3 player.
Far Away by Jose Gonzalez
Red Dead Redemption's masterful rendering of the spaghetti western vision is complete, at all angles. From sound effects to visual artistry, it succeeds without a misstep - and the music is no exception. For ninety percent of the title, we enjoy twangy, distant, iconic music (that deliciously surges when action's afoot), but this particular song is almost directly responsible for one of the most memorable moments of the year: John Marston's patient, rolling ride into old Mexico.
Word to the wise: don't get off your horse.
Fly Me To The Moon (Remix) by Helena Noguerra
Bayonetta is controversial, sometimes offensive, always outrageous and a game with a uniquely purposeful feminine bent. Everything in the title - with mechanics and tropes borrowed from the best in action gaming - is informed with a bit of female flourish. Bayonetta's double-jump is accompanied by butterfly wings, she cracks open magical barriers with a blown kiss, and her lock-on indicator is a pair of neon-red lips.
This extends to a very, very different original soundtrack, with everything from tempo to arrangement chosen to continually provide a sense - whether a gentle barroom blues or a raging boss fight blitz - of ladylike acknowledgment.
Desert Wind by Mark Morgan
New Vegas announces its wonderful old-timey soundtrack right off the bat - but if the player decides to turn off their Pip-boy, what they get is an entirely different, and equally decadent treat. The radio stations in New Vegas are fantastic in their own right, but if you spend the whole game listening to them you will have completely denied yourself the fantastic ambient stylings of Inon Zur (Fallout 3, Prince of Persia '08) and Mark Morgan (Fallout, Fallout 2).
With songs that directly recall classic Fallout music and ever-creepier, infinitely-lonely tracks for every area (and variations for day and night), Fallout: New Vegas easily boasts one of the most impressive and immersive soundtracks of the year.
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Pairbond by Gary Schyman
Like New Vegas and BioShock before it, BioShock 2 employs world-appropriate but disturbingly ironic music of a bygone age. Blowing the heads off splicers to How Much Is That Doggy In The Window and (personal favorite) Bei Mir Bist Du Shon is still a fantastic juxtaposition, but BioShock 2 excels and - to me - outpaces every other game this year via the same route as the runner-up: with incredible instrumentals to compliment the space between those classic tunes.
I don't know why it is that BioShock 2's score affects me so - I don't have much of a vocabulary, when it comes to explaining the mechanics of music - but I can tell you that Gary Schyman's music, for some reason, almost manages to move me to tears all on its own.
Tip to tail, the music of BioShock 2 is - objectively - fantastic. I name it the best soundtrack of the year not because of its measurable qualities, listed by bullet-point and tallied via spreadsheet, but because of how deeply it affects me, personally.