I have lots of favourites but I feel like most will be suggested by others so I’ll go for one that isn’t as popular: Monaco: What’s Yours Is Mine. The whole soundtrack is amazing start to finish. Some of the best piano tunes to ever be in a video game, and a super under-appreciated game. It should’ve been way bigger than it was. Unfortunately it’s being tainted by its less-than-stellar sequel which came out recently. Don’t play the sequel, but definitely play the original. It’s fun solo but it really shines co-op with a friend (or three). It supports up to four players and I’d definitely recommend co-op if you have the means. I used to play it on 360 with a friend of mine and it’s some of my most fond gaming memories. It’s not a difficult game to figure out but it’s not an easy game to get good at. But all that is to say, play the game, or if you can’t, absolutely listen to the soundtrack. It’s awesome.

Batman the Videogame on the NES, FFVII, the entire WipEout series, original soundtrack for Crazy Taxi, Sonic the Hedgehog 3/Sonic & Knuckles, X-Men on the Sega Genesis, WET, Omega Boost, Zone of the Enders, BurnOut 3: Takedown, Mobile Suit Gundam: The Lost War Chronicles, Roommania #203, Street Fighter Alpha 3, Mega Man 2 & 3, Doom, SSX Tricky, Xenogears.

Icewind Dale

The Outer Worlds

Deus Ex

For entire OSTs: any of the FromSoft soulslikes, Lies of P, Diablo 2 + Lord of Destruction, any Monster Hunter game really (especially the ones in Pokke village, so Freedom 2 and Freedom Unite/Portable 2nd G), Hollow Knight, currently enjoying Ridge Racer's (PSP) soundtrack for working out, Deltarune and Undertale, Tony Hawk's Pro Skater 3 and Bboy the Game for some perfect-vibe licensed music...

Disco Elysium's is my current favourite OST, but the game itself might also just be current favourite game.

As for individual tracks, I like the Spanish guitar portion of "The Drive/First Contact" off the RE4 soundtrack. Back when I started out with breaking ("breakdance"), think 2008/9ish, we had a remix of that specific part of the song that would play often - I've got very fond memories of that. The song itself, its remixes and the remake versions actually are some of my favourite running music for the gym.

Here's the remix in question if anyone's interested: https://youtu.be/SMWV-3pbcLE

As for one favourite individual song from each of the aforementioned OSTs:

• Hunter's Dream (Bloodborne)
• Shadow Flower (Lies of P)
• Rogue (Diablo 2) + Lord of Destruction intro
• Pokke Village theme (MH Freedom 2/Unite/Generations (Ultimate)) and Mizutsune theme (first fought in Generations)
• Mantis Lords (Hollow Knight)
• Rotterdam Nation - Remix (Ridge Racer)
• Ruins (Undertale) and My Castle Town (Deltarune)
• I'm a Swing It by House of Pain or Ace of Spades by Motörhead (THPS3)
• Don't Sweat the Technique by Rakim or Apache by Incredible Bongo Band (Bboy the Game)

This was lots of fun to think about. I hope anyone reading can find something of value here. :)

Faster Than Light

https://www.youtube.com/watch?v=WFkGjEut9U4

Going through the lists everyone has posted I realized I’m surprised I haven’t seen anything from the Medal of Honor franchise posted. I only ever played Medal of Honor: Frontline because a roommate had it, but it was scored by Michael Giacchino, who also composed the scores for the original Medal of Honor, Medal of Honor: Underground, Medal of Honor: Allied Assault, and Medal of Honor: Airborne. Giacchino won awards for the first four soundtracks in the series and it was his video game work that caught J. J. Abrams’s attention and led to him scoring Alias and Lost, starting his TV and movie career.

Also, if you enjoy soundtracks, you might like StreamingSoundtracks.com. It’s been around since 2001 and features soundtracks from movies, tv shows, and video games. It’s a free streaming radio service with the playlist queued by listeners along with limits on how frequently any track can be played, so you won’t hear repeats very often. A manager at a past job played it most days. One year someone took advantage of the “No repeats” rules and played all the horror tracks throughout October so that on Halloween there were basically no horror tracks available, angering many when they realized what had happened. I found it one of the more impressively planned and executed pranks I’ve personally witnessed.

A lot of great ones have already been mentioned: (Old School) Runescape, Bastion, Bomb Rush Cyberfunk, Braid, Castlevania: Symphony of the Night, Celeste, Chrono Cross, Chrono Trigger, Crypt of the NecroDancer, Cyberpunk 2077, Deus Ex (2000), Donkey Kong Country 1 and 2, Doom 2016, Earthbound, F-Zero GX, FTL, Hades, Halo, Hi Fi Rush, Hotline Miami, Jet Set Radio and JSR Future, Kirby (any of them), Legend of Mana, Mother 3, Neotokyo, Nier: Automata, Pizza Tower, Quake, Ratchet & Clank (any of them), Risk of Rain 1 and 2, Shadow of the Colossus, Shantae (any of them), Shatter, Shovel Knight, SimCity 3000, Sonic 3 & Knuckles, Sonic Adventure, Sonic Adventure 2, Sonic CD (Japan), Sonic Mania, Sonic Rush, Stardew Valley, Tower of Heaven, Transistor, Trine, and VVVVVV

Here's some that haven't been mentioned yet:

Ape Escape

• PS1 game, soundtrack by icon of the 90s era Japanese House music scene, Soichi Terada. All of his tracks are bangers. I've seen the soundtrack described as House, Jungle, and DnB. Probably my favorite game soundtrack ever, but that's probably nostalgia talking.
• Soundtrack: Game Rip, Fan Remastered or Originape Soundtrack/Official Soundtrack CD (Note: Terada has released his own remastered tracks in the 2020s since the Originape CD, but they are split over multiple albums and I'm too lazy to track them down at the moment)
• Standout tracks: Peak Point Matrix/Galaxy Monkey, Primordial Ooze, Thick Jungle- Woods, Dark Ruins, Crumbling Castle, Specter's Factory- Outside, Time Station

Final Fantasy Legends III / SaGa 3 Jikū no Hasha: Shadow or Light

• Originally on Gameboy, Japan only remake on DS (fan translation patch exists). This is really a game in the SaGa series, but was released under the Final Fantasy name internationally for marketing purposes. Characters don't have levels, their stats change based on what they do. Enemies also drop meat and parts that you can apply to a character to change their class, with some unique mechanics for different types. Features a time traveling stealth bomber, THE TALON, that may possibly be the only airship in gaming with a working toilet. DS Remake is generally better, but it kills overworld exploration by turning it into "select an adjacent POI" instead of free roam.
• Soundtrack: Gameboy, unfortunately I can't find this version with track names. DS Remake
• Standout Tracks: Setting off into the future, Holy Ruins (a criminally underused track in game), Oasis (has some odd animal sfx added in the middle in this remake version), Stronghold (imo, one of the best dungeon themes in gaming), THE MOTHERFUCKING TALON THEME (this is the GB version, as the DS remake loses the countermelody which imo weakens the track)

Ghost Trick

• Originally for DS, has been remade for just about everything now. It's effectively a VN with fancy gameplay segments, and the music really helps carry the whole thing. Beautifully done sprite animation too.
• Soundtrack: DS, 2023 Remaster
• Standouts: Ghost Trick, Lynne- Red-Headed Target, Awakening, The Mystery Surges Ahead, Cabanela - Lanky and Loose Lawman, Jowd- Fated Jailbird

Gunpoint

• Old indie sidescrolling noir stealth game. The main gameplay revolves around puzzle solving by hacking buildings' electrical systems. Hook that lightswitch up to the secure door lock. Rig that motion sensor up to short out a power plug and take out a guard instead of it just turning on the lights. Delightful jazz soundtrack.
• Soundtrack
• Standouts: Security, Circuitry and You, Subterfuge Shuffle, Cold Halls and Footfalls, The Five-Floor Goodbye

Megaman ZX

• DS sequel to the Megaman Zero series for GBA, but now with a Metroidvania twist.
• Soundtrack: ZX Tunes- Official remix/rearrange soundtrack
• Standouts: Innocence- 90s style anime opening, Green Grass Gradation, Rockin' On, Brilliant Show Window, Cinq Ville - C`est Notre Espoir, Gauntlet

And I've spent way too much time on this comment. Here's more not mentioned so far that I don't have time to do a writeup for: Skies of Arcadia, Super Meat Boy, Tomba 2: The Evil Swine Returns, Uplink, XCOM (Original), XCOM (Reboot), XCOM 2 (Reboot)

My favorites:

• Superbrothers: Sword & Sworcery LP
• Hyper Light Drifter
• Into the Breach
  • Basically, Ben Prunty does amazing work. FTL and others have already been mentioned

Pillars of eternity 1 and 2, octopath traveler, Chrono trigger, streets of rage, sonic 1-3, and doom eternal were all pretty amazing

Journey

The game is a piece of art and the soundtrack plays a big part of it.

I’ve seen a lot of good mentions here already but haven’t seen Nier: Automata yet. I thought the game was overrated, but there are a lot of great audio tracks

axiom verge 1 and 2 - good in different ways. i liked to put first on for long coding sessions, and the second expanded in interesting directions
transistor & pyre & hades - all scored by Darren Korb, but different genres. from first moment, transistor has such vibe in big part thanks to it's soundtrack, and is among my all time top games
Nier automata - won awards for it's soundtrack, clear to hear why
crypt of the necrodancer - rhythm game should have a good soundtrack, and it does
witcher 3 - the battle themes - like "silver for monsters" are unique style of action-y folk, very rare to find anything else that scratches that itch.
dusk - awesome and dynamic metal
risk of rain 1&2 - first game itself didn't click with me, but, oh man, that solo in "Monsoon" get me every time, and "Coalescence" is poignantly melancholic in a similar way to "song of the ancients" from nier - which is (a very high praise and) much more expected from nier's type of game than a "risk of rain"'s. "face of the deep" from ror2 from second is head-spinning in a good way
no man's sky - it introduced me to 65dos, for which i'm very grateful. that soundtrack is still probably my favourite album of theirs noita - low-key psychedelic, very vibes
unreal tournament 99 - and i think it's not just nostalgia, because one of my favourites - "mechanism 8" - is from the map i almost never played
"Ultrakill" - good soundtrack, and yet cyber grind tracks from guest composers are still my favourites. "The cyber grind" in particular is just exhilarating

individual tracks i overheard from games i haven't played:

old "touhou project" games - their soundtrack is full of gems, even if it's not my genre of game
system shock 2's "Med Sci 1" and "Engineering" - very nice, i want more of that and can't find
"Clair Obscur: Expedition 33"'s "Lumière" - made me wishlist the game just for having it
"Final Fantasy IX"'s "You're not alone" - very nice melody i like to whistle sometimes. considering playing it just for having it
original "Castlevania"'s "Vampire killer" rocks

Katamari Damacy.

Na na, na na na na na, na na, na na na na na na...

Outer Wilds - The way this game uses music and ambient sound to foster a sense of wonder, dread, excitement, sorrow, and much more is nothing short of incredible. Probably the best game I have ever played.

Subnautica - Equally impressive use of music as Outer Wilds, many of the same feelings, but a much much different vibe and genre.

Return of the Obra Dinn - A grim game where the soundtrack with church bells and heavy string instruments really underscore the mood.

Marathon 2: Durandal has only one song, the title theme, but I do like it as a good "get one pumped to play the game" song! From the band Power of Seven.

EDIT: Also, while I don't know if I'd want to just listen to it alone outside of the game

never tried

Jets'n'Guns has a soundtrack that works well for the game. Done by the band Machinae Supremacy. One of the game's levels is set in a future Machinae Supremacy concert.

Also, Bandcamp link for the soundtrack: https://machinaesupremacy.bandcamp.com/album/jetsnguns-soundtrack

Lots of good choices in the comments already so I'll start with what I'm currently obsessed with: Project Wingman and its DLC, FRONTLINE-59.

OTXO is also great if you're looking for some heart-pounding action, both in-game and to listen.

Neotokyo, the old source mod has two fantastic cyberpunk albums for its OST: GSDF and NSF. Both have their own unique style instead of trying to imitate the classics which is a huge plus.

I've seen Drakengard being mentioned already so have some heavenly sounds announcing the end of the world. If you'd like something that doesn't cause as much mental damage as the first game, Drakengard 3 is your friend! It's still possesses the mad undertones of the original and some of the boss music can be take it or leave it for some people as the heavy electronic vibes can clash with the medieval fantasy aesthetic (I love them all personally) but it has some absolute bangers as well.
Here are the usual suspects when people talk about this title:

• Black Song - surprisingly light considering when it plays in the game.
• The Final Song is a backing track for an 8 minute boss fight with no checkpoints and no place for mistakes (1HP through the whole thing baby!).
• This Silence Is Mine - if you want to listen to someone who've been through hell and back singing asking for the sweet release of death. It's raw, it's emotional and it perfectly encapsulates all the themes of the D3's protagonist, Zero. Still one of my favourite OST songs of all time despite listening to it for at least a decade now.

Last but not least, Shantae and the Pirate's Curse. It's filled with fun and energetic tracks which fit perfectly with the light comedic tone of the series. Jake Kaufman's masterpiece and probably my favourite OST in the series.