Great topic here! I also love a bunch of game music and have tons in my music library. I wholly agree on Outlaws and Bastion! Some of my choices:
Mirror's Edge - I've said this before in my standouts of 2016 but I'm a complete fanboy for Solar Fields. Both games' score get heavy rotation for me.
Halo series (and by extension Destiny) - Marty O'Donnell is a master. All of the scores written by him are in my library and love them all. I really like the Halo ODST score in particular since it's a departure from the standard Halo fare. More subdued and introspective that matches the tone of ODST. The Destiny score also gets heavy play. While Marty isn't with Bungie anymore, C. Paul Johnson is carrying the legacy admirably.
Elder Scrolls - Just about anything Jeremy Soule does I'll be listening to. I have all of his Elder Scrolls scores and the ones from Guild Wars. Skyrim is an easy choice for best, but I'd say Oblivion gives it a run for it's money.
Remember Me - This one is a curiosity but fits the game well. Bombastic when it needs to be and melodic when it counts. The score is laced with 'glitching' that suits the theme of memory reconstruction. Excellent mix of traditional orchestrations with electronic mxing.
World of Warcraft - Say what you will about the game or Blizzard, but they do good work. Their overtures for the login screens really set the stage for the journey to come in each expansion. Themes from various points are woven into a tapestry that carry over to when you're playing across the game. Favorite track is either "Sacred" when it was redone a few expansions ago or the suite for Karazhan. I do admit that the Legion score has some questionable choices, but overall they've done well.
Silent Hill - Any of the scores by Akira Yamaoka are amazing. SH2 is probably my favorite. It's atmospheric and quite creepy in parts. The spoken parts of SH3 also suitably creepy. The wife and I actually play a random selection of these scores when we're playing Arkham Horror LCG.
Other standouts: Bioshock I have both the score and all the licensed songs from the game in one playlist. Coordinated with forum folks to get the track list just right where each song appears in the game. Overwatch What I said above with WoW applies here. Katamari Damacy What can be said here? Just unplug your mind and enjoy!
So many good scores and soundtracks to list them all!