I tend to fall in love with soundtracks where I have an emotional attachment to the game, so my all-time favorite is the soundtrack for Final Fantasy Tactics. I bought the two-CD set forever ago and it still cracks me up that my devices read Japanese better than I do.
Games where I have 100% loaded the game expressly to hear the music include The Sims (1), Tropico (1), Chocobo Racing (yes, there was an actual PSX Chocobo Racing game, not the FFVII mini game within the game), and Civilization VI (I'm terrible at the game, but dang, the music is great).
I do like the Stardew Valley soundtrack a lot, but I hear it a lot either when playing or watching Youtube videos about Stardew, so I haven't gotten to the point where I want a standalone copy of the soundtrack. Yet. I might get there in a few years.