Unreal Tournament 2004 is my favorite OST by number of tracks that I love. It has several absolute banger tracks, like Morpheus3, Hyperblast-Redux, Convoy, Atlantis and Level11. I often keep a number of those tracks on my playlists.
Although I don't like the game itself much (I suck at it), Cloudbuilt also has some awesome tracks, my personal favorite is Aurora. The guitar chords at the apex is just heavenly.
Rebel Galaxy has some very nice stuff to feel like a 'merican space cowboy, it's mostly licensed music as far as i can tell.
Assault Android Cactus usually gets your blood pumping hard, which is great because the game is a pretty fun twin-stick arena. The tracks Assault and Licorice are just ๐จโ๐ณ๐๐
... Come to think of it, looking at my list, I'm a real sucker for sci-fi OSTs, huh. I do want to make honorable mentions to Mass Effect (mostly 2), Skyrim and Smash Bros. Ultimate
