Chrono Trigger, of course. Some legendary tracks in there that transcend their limitations. The Deadbolt OST, Katana ZERO's OST. Nier Automata, Street Fighter V.
Oblivion and Skyrim as well. Really there are too many excellent game soundtracks to possibly list in a comment
AI War / AI War 2 - Pablo Vega
Dragon Quest series - Koichi Sugiyama
Final Fantasy VII - Nobuo Uematsu
Final Fantasy XI - Nobuo Uematsu and others
The Last of Us - Gustavo Santaolalla
Medal of Honor - Michael Giacchino
Offworld Trading Company - Christopher Tin
Stellaris - Andreas Waldetoft (I didn't notice that this game was just an idle clicker for a long time because the music was so damn good)
Tidalis - Pablo Vega (especially the piano versions)
A Valley Without Wind - Pablo Vega
Sonic Mania is one I always fall back on, the tracks are fun as hell and I don't think there's a single one I don't like. Shoutout to the Studiopolis act 1, Studiopolis act 2, and Press Garden act 2 tracks.
Witcher 3. Surprised I haven't seen it yet. The vase game is totally the best in my ears, especially the Skellige soundtrack but the expansions are also a blast!
Maybe this answer is cheating, but Smash Bros Ultimate has the best soundtrack of all time. It's got over 1000 tracks, including some of the most recognizable video games music of all time, and some amazing remixes.
The legend of Zelda always had the most iconic themes (personal favorite is Dragonroost island from Wind waker, but they really all have some amazing music).
I also listen to the Chrono Trigger and Octopath traveler OSTs on my free time.
There are many I love but I gotta go with Evergrace. I know it's great because I've never played the game and am still always putting this one on.
Though if best means best in the context of the game then I'd probably have to go with Sim City 3000. The OST sounds fantastic and really captures the vibe of a city that is wires and plumbing sprouting into something living and breathing.
-Assassin's Creed Odyssey: Legend of the Eagle Bearer, The 300, Odyssey(Greek version).
-Assassin’s Creed Origins: Main theme, Return of the Medjay, Bayek of Siwa, The Shimmering Sands, The Battle of Krokodilopolis.
-Assassin’s Creed Valhalla: Main theme, Shadow Walker, Out of the North.
-Battlefield 1: Nothing Is Written, Avanti Savoia, Hunted, Cape Helles, Zeebrugge Harbour, Sand and Dust, In the Name of the Tsar, Grace and Glory, Sword in Hand.
-Battlefield V: Legacy Theme, Under No Flag, Far From Home, Spitfires, Rotterdam In Ruins, Devestation, I Vow To Thee My Country
-Battlefield 4: Warsaw Theme, Stutter theme, Jin Jie's Revolution, A Theme For Kjell, Fishing in Baku.
-Battlefield Bad Company 2: The Storm, Snowy Mountains, Operation Aurora.
-Call of Duty: Modern Warfare 2: Opening Titles, Extraction Point, Coup de Grace, Chain of Command, Esprit de Corps, Protocol.
Lots of good answers in this thread. Thank you for wording this in a way that allows me to post more than one answer, lol. I'll name a few I didn't see reading through here:
Talos Principle
System Shock remake
Metroid Prime
FF VI pixel remaster (all the pixel master OSTs are great, but VI got the most love)
Its insane how much the soundtrack makes this game. Probably the best use of music in a game outside of musicaly focused games. It's not just a few moments where the music is used effectively, its every single moment.
Chrono Trigger, Chrono Cross, FFVI and Xenogears top out the list for me. You can't beat them. If you want something a little off the beaten path, the soundtrack to Ruiner is pretty fucking rad.
ehh fuck it, im gonna say its space funeral. It's a bunch of music lifted from the 70's BBC sound archives. Its clearly not the most expensive or masterfully produced soundtrack, but that's nowhere near the point. Its the dichotomy between the ugly visuals of scum vullage and the melancholic first track that persuaded me to stick around, and then it went in all sorts of different directions over the course of an hour long game.
Golden Sun. Despite some other masterpieces, I still maintain that the trilogy of games represent Motoi Sakuraba's magnum opus. Nothing else from the life of the GBA came anywhere close to even touching it, and even now tracks will randomly get stuck in my head, more than I can say for any other game soundtrack I've ever heard.
Subnautica 1 and below zero (super calming to listen too and also fit the vibe really well)
No man's sky (music fits the atmosphere of the game really well)
Payday 2 (very good kinda electronic songs)
Superliminal (very good calming piano)
Boneworks and bonelab (these are absolutely phenomenal like electronic tracks, Michael Wyckoff fucking nailed the sound track for both games and both are perfect)
Red dead redemption 2 (the entire game is perfect and so is the sound track) (haven't played the first one)
But I swear it's always the tiny little indie games that hire some guy off of fiverr or something like make some masterpieces.
I'm going to go deep cut here with Outlaws (Lucasarts, 1997). If you don't have time to listen, imagine an Ennio Morricone Spaghetti Western score, but with John Williams-like melodic leitmotifs. It was done really well, and lived on my various MP3 players for many years.
I’ll limit myself to recently released games. Blasphemous and Dead Cells have really good soundtracks. I think Axiom Verge’s soundtrack is very cool too but not something I listen to often.
ULTRAKILL has a phenomenalOST. Fast paced and very breakcore for a stupidly fast FPS set in hell with crunchy PSX graphics, but still with these weighty almost sad moments of calm.
On the complete other end of the spectrum, Project Moon. They've done two turn based deck builders that force you to slow the fuck down and read, and are also a fan of using "actual" songs (made a group called Mili) for boss fights that are slow, melancholic, and ReallyGood. they've also done the main themes for BothGames but idk if they strictly count as 'OST'.
Also I know Persona gets all the love, but the mainline SMT series has some fucking bangers too, just more in the Club and/or Hell yeah tubular bells genre
Though Castlevania: Lords of Shadow was a divisive game, its art direction and soundtrack are incredible. Oscar Araujo’s score combined with some great vistas and setpieces elevates the mediocre gameplay to actually make this game one of my favorites.
I also loved the songs from Death Stranding. Low Roar’s tracks fit the atmosphere of the game perfectly, and the few tracks from other artists really stand out in a nice contrast to Low Roar’s calmer feeling.