Pretty self explanatory topic. I'm sure everyone has their own list of games and soundtracks they feel fall under this category, so have at it!
The first that comes to mind for me is Castlevania Curse of Darkness. The gameplay is very solid, and the OST is even more so. I feel like it didn't get that much attention, because it was a 3D Castlevania title, which have previously (and afterwards) been notorious for their mediocre quality. But this one was the exception out of the rule. I won't claim it's a masterpiece of a game, but it's easily a solid 8 / 10 in my eyes. Great gameplay, solid controls, lots of customization (weapon crafting and Innocent Devil evolution), and lots of exploration as the Metroidvanias are known for. It's got average ratings across many game sites though, unfortunately.
Sample a couple of the tracks from the game, the atmosphere is just top notch.