This thread is more about relatively unknown (not-rated) soundtracks than underrated soundtracks (and you also have people mentioning examples like Nier and Bastion, which makes no sense in either case).
What I would say is underrated is Mass Effect 2's soundtrack as people love to crap on ME2's soundtrack to put ME1's on a pedestal.
It's mainly frustrating when someone says ME2 has too much of a orchestral sound (usually implying that orchestra music is less interesting, a dumb sentiment in itself) versus ME1 having more electronic. The problem is that's false and comes off as a meme that got its legitimacy from blind (deaf?) repetition. ME1 features a lot of orchestra sounding music and ME2 a lot of electronic music, and in both cases they mostly feature music that sounds like both. Consider how important tracks in ME1 like
The Presidium,
Sovereign's Theme, and the climax's
Victory are pretty much entirely "orchestral" and that most other themes are like the
Main Theme and
Normandy in how they rely really heavy on traditional string and drums in addition to the electronic sound like you'd expect of any "hollywood" (this is suppose to be a bad thing for some people) soundtrack. It's worth noting mostly the same people, namely Jack Wall, worked on both. I think it may be the case that people judge ME2 entirely on its orchestrated title theme and imbue it with their
hardcore gamer victim complex because how they feel about the ME1->ME2 transition in other respects, and this is despite the instances of the main theme of ME1 being mostly orchestral.
Now, to actually get on point, what I appreciate most about ME2's soundtrack is how there is a theme dedicated to each character for their recruit and loyalty missions, in addition to central missions getting their own themes. What's special about these themes while they stretch out for the entire mission, they actually change several times (e.g., multiple background and battle themes) and get more intense with as you progress through the level (of course, you can't see the dynamism listening to the single tracks on the official OST). To name I think show this off well and to follow up on my counter-criticism above:
Thane,
Samara, and
Tali (probably my favorite example of this).
The songs outside these mission themes are in no way lacking either.
Illusive Man's theme is probably one of the very best themes in the entire series right next to the iconic galaxy map theme,
New Worlds. The
Upper Afterlife theme is the most memorable of the series's club themes, used in the series's most important club.
Suicide Mission is actually a fantastic theme, probably one of the better uses of the series's main theme (which itself has been usurped by New Worlds). I'm a big fan of ME2's atmosphere, champion it as the best atmosphere of the series and one of the best of the previous generation, and the music obviously plays a huge role in that. The game itself is a flawed cover-shooter, but the overall atmosphere, the TV show momentum with its episodic missions, and the character-centric heist film approach complementing BioWare's style of characterization makes it a engaging game.
Let this meme die.
. 
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