Mine is Skyrim.
I always come back to its music. It’s uplifting, telling me that everything’s gonna be alright. I have its songs in many different playlists.
Unreal Tournament and Deus Ex both come to mind. Alexander Brandon was involved in both and his work is absolutely amazing.
If we talk specific singles, though, it’s Morrowind (Nerevar Rising), Control (Take Control), and, recently, Baldur’s Gate 3 (Raphael’s Final Act). Morrowind’s tune is so ingrained in my mind that it’s my to-go whenever I get my hands on a keyboard.
Great choices there! the Unreal Tournament and Morrowind soundtracks have been stuck in my head for a long time now. UT in particular – I listen to the full album about once a week while I’m working.
I never played Deus Ex when it came out, so I don’t have the nostalgic attachment for that one, but I just discovered this a few days ago: https://alexanderbrandon.bandcamp.com/album/conspiravision-deus-ex-remixed. I bought it and am giving it a full listen today. Highly recommended; probably that much better if you’re a fan of the game.