I have several OSTs on my MP3 player, and people always give me puzzled looks when they notice it. I'm glad I can share some here.
Now I'm pretty sure most of the music I'm gonna post was already posted, but hell, I'll post it anyway, because I think they deserve it.
So, first off, let's start with something calm. Those musics are from the game Endless Space.
Here's a link where you can listen to most of them:
https://gameaudiofactory.bandcamp.com/album/endless-space-soundtrack
I suggest listening to "Deafen Dawn", "Flawless Theory", and "Beneath the Strings", at least. I think the entire OST is good, with a few amazing tracks, but that's just my personnal advice.
It's a turn-based strategy game set in space, and the chill music really fits. Also, a strong factor that made me like the music, I think, is that the music scales depending on your actions. If you look at the entire galaxy, it will be very calm, but when you zoom on a solar system for instance, it will add a few instruments. It doesn't sound much, but I guarantee it really adds to the atmosphere.
In the same vibe, the same composer made soundtracks for a game called Endless Legend. It's also pretty chill, but this time it's set in a Fantasy setting. It's worth taking a shot.
http://gameaudiofactory.bandcamp.com/album/endless-legend-soundtrack
I'd suggest to try "Dusty Strings" or "From Dawn To Dusk". I'd like to suggest more, but I'll just let you work your way through the playlist.
Now, for something a bit more energic: The Ace Combat: Assault Horizon music has amazing tracks. Some tracks are just "ambient" tracks with no real music, but some are really packing a punch. Here's a few examples (Sorry, couldn't find non-Youtube links):
If you liked those, I'd suggest going for "Dogfight", "Fighter", or "Shall Defend".
I'd like to name a few other soundtracks, but I feel like I'm going to spam the board if I do. So... Yeah.