Arma 3 has one of my favorite soundtracks. Very fitting with the near-future thing they tried.
https://www.youtube.com/watch?v=9jcehv7VBXsXCOM 2012 has an excellent set of music too, but my favorite are the base soundtracks. They change based on where you are in the story. Act 1 is kinda muted, a calm before the storm kinda sound. Act 2 is cautious and calm, which is pretty much how the player feels by the midgame. Act 3 has more base, and generally louder and more action-y than any of the others which fits with the high stakes game you play by the end of the game.
https://www.youtube.com/watch?v=xd2n7IJEp8Qhttps://www.youtube.com/watch?v=T6bXcD0VBzIhttps://www.youtube.com/watch?v=klE6VqLGNP0Rollercoaster Tycoon 2 has a great theme song. Instant nostalgia.
https://www.youtube.com/watch?v=oENUA2B1_uMThe Rimworld soundtrack is pretty great in general too. The lead dev guy has this vision for the game that's it's gonna be something like a space-western, and the music reflects that. Sometimes it doesn't really fit the visuals or gameplay, but taken on its own the music is very good.
https://youtu.be/ybVJAtRHeCIStalker Misery mod has some pretty good music on its safezone radios, but I can't find them on youtube since I don't actually know what they're called. You'll just have to trust me when I say that they are exceptionally fitting, although I suppose they aren't original soundtracks.
Oh yeah, and pretty much every Halo has an awesome soundtrack. While 3 is probably the classic and definitive example of it, I'm gonna post Reach's OST here because it's pretty unique for the series and also very good.
https://www.youtube.com/watch?v=53_CUSmf8fQ
