I have been gaming for 35 years and I have strong opinions about this. Fight me.
MY TOP TEN (unordered because ranking these against each other is genuinely cruel):
Chrono Trigger — Yasunori Mitsuda composed this while hospitalised with a stomach ulcer he developed from overwork. The result is one of the most emotionally varied and technically accomplished game soundtracks ever written. Corridor of Time alone.
Streets of Rage 2 — Yuzo Koshiro created this on a PC-88 using self-written composition software. It sounds like nothing else in gaming. The fact that it runs on Mega Drive hardware is still baffling to me.
Final Fantasy VI — The Opera scene. That's it. That's the whole argument.
Halo: Combat Evolved — Martin O'Donnell and Michael Salvatori built an entire sonic universe from a Gregorian choir and a percussion section. The main theme is one of the most recognisable pieces of music from the 21st century.
Shadow of the Colossus — Ico was good. SotC is transcendent. The music shifts from pastoral serenity to overwhelming orchestral terror in the time it takes a colossus to turn around.
Celeste — Lena Raine. Modern classic. Already mentioned in my reviews but it bears repeating.
Nier: Automata — Keiichi Okabe. Multiple vocal tracks sung in an invented language. The final chapter music is one of the most emotionally devastating things I have ever heard in any medium.
Donkey Kong Country 2 — David Wise is a genius. Stickerbrush Symphony makes grown adults weep. The SNES had no business producing audio this good.
Persona 5 — Shoji Meguro. The swagger. The jazz fusion. Last Surprise. Need I say more.
Tony Hawk's Pro Skater 2 — don't laugh. That tracklist shaped an entire generation. Guerrilla Radio comes on and my legs remember the Warehouse level.
What have I missed? What would you add? And please, someone, tell me I'm right about Streets of Rage 2.