Purely based on nostalgia: Ocarina of Time
Some great music. I wonder if these games wouldāve had the same impact without themā¦
I couldnāt believe no one had linked OOT! I was about to, before I saw this. +1,000. Favorite of all time.
Skyrim, Destiny, Halo, The Simsā¦ A lot of the time I just play them to listen the soundtracks.
Great selection of soundtracks around here! I think I like too many to make a list myself, but the first I had in mind was the NES game Ā«Castlevania II - Simonās QuestĀ» soundtrack (maybe a bit of nostalgy in my choice, but the tracks are all good, standing by themselves). 
https://youtu.be/obapGp8ih0M
Oh Yeah, Gareth Cokers work on Ori is absolutely great, Zimmers Modern Warfare II theme too.
My all time favorite is the Grizzly Hills music:
On a side note, if you enjoyed Jeremy Souleās stuff for Morrowind then check out Morrowind Rebirth:
Iāve contributed a handful of tracks along with a couple of other talented composers (The first and the last track in the video is from me).
Battlefield 1942!
GTA3 Main theme 
The Sims has some awesome music! The Sims 2 is my personal favorite. So personal and fun.
Fez has got to be one of them for me- or indeed anything by Disasterpeace/ Rich Vreeland
Oh man! What a classic! 
Ok, you have asked for it:
Out of so many, probably the entire soundtrack for Homeworld 1 by Paul Ruskay, because it shaped the mood of vast emptiness of space so effectively and went all the way against the establishment: while most people back then used to compose powerful symphonic tracks or dynamic electronic for space games, Ruskay mixed ethnic drums with ambient sounds.
Hereās āImperial Battle Musicā:
That would come really close to my number one. Fantastic for innovative and conveying the loneliness of space. I prefer the first to the second, though.
Indeed, and I may also add that original H1ās soundtrack is better than H1ās remastered one. Maybe itās just a psychological effect, but remastered edition ālostā something that was present all these years ago, back in 1999 :).
I agree, probably due to the compromises they did to unify both games into the Homeworld 2 engine, which altered some cool things about the gameplay. But I still think itās a fantastic game and enjoy playing it from time to time,
I remember playing homeworld as a kid on some demo disk, didnāt understand anything because the demo lacked a tutorialā¦ But the soundtrack was indeed mesmerizing!