Hard to pick just a few tracks, but some of my favorite original soundtracks are:
Nier Gestalt
Persona 5
The Witcher 3
Stellaris
For licensed ones, I particularly like Grand Theft Auto: Vice City and Life is Strange: Before the Storm.
Umm...let's try going through the ages
I was like 4: Gizmos & Gadgets ending theme. I'm pretty sure it's one of the few original songs The Learning Company did, there's very few clips of it because it was before even .wav was commonly used for music (due to file-size issues), it's a really cheerful song
Early Childhood: Space Quest 6 Main Theme is a bop, also Human 3 from the base game of Warcraft 2, and Orc 6 from the expansion. I miss the less ambient and more intense Warcraft music.
Listened to a lot of anime music around wc3's time...but peak music at the time was Sonic Adventure and Sonic Adventure 2, plus Phantasy Star Online thanks to the dreamcast. Especially 'Pray For IDOLA The Holy'. Grandia II also giving the Ver.3 and Ver.4 boss fights.
Nowadays with great music being everywhere? Probably Persona 5 in general.
I regularly listen to the Subnautica OST while working.
Maximum Anarchy's soundtrack
Oh, man. So many good ones, especially in the indie space. Celeste and Eastward both have spectacular, eerie, synthy soundtracks that are wonderful. Another indie game, Flinthook, has a theme song that absolutely slaps. Katana Zero had a rad OST, too.
Undertale goes without saying.
Obv Nintendo games, especially ones composed by Koji Kondo, have amazing soundtracks. OoT is one of my favorites, but there are many good ones. Basically any classic Zelda, Mario, Donkey Kong, etc. Even the newer games, like Mario Odyssey and TotK, have great soundtracks. While not first-party Nintendo, the first couple gens of the Pokémon games' music will live in my brain forever more.
I'm also gonna shout-out Grant Kirkhope because he is a genius and basically composed the soundtrack to my childhood. You know him from Banjo-Kazooie, Donkey Kong 64, Mario + Rabbids, and many more games. They aren't soundtracks I'd walk around listening to on Spotify (probably), but in the context of the game world, they are perfection.
I could go on, but I'm just gonna shout out my faves and walk away before this becomes a giant wall of text.
EDIT: Oh lord, how did I forget Halo 2's theme (Mjolnir mix)?
I have five fingers on one hand so here are five favorites:
Subwoofer Lulldlaby from Minecraft, Basically all of the Alpha album Minecraft music
The only game soundtrack I ever recall make a point of listening to is from Faster Than Light, it's some really good ambient music.
Favorite instrumental soundtrack themes are from the early Silent Hill games. The theme from SH1 and the Theme of Laura from SH2 are classics.
Favorite lyrical songs used within the game for plot: Alan Wake has characters in it who are 70s rock gods called Old Gods of Asgard, and the creators got a group named Poets of the Fall to actually write the two songs that were featured from the game and they are actually really great songs. Poet and the Muse is a plot point and Children of the Elder God is played during a really good sequence.