Thunderforce (Lightningforce) 4 (MegaDrive/Genesis)

While not actually an OST, I recommend "The Megas" works. They made a number of albums revolving around Megaman and Castlevania.

The Megas: Man on Fire

Three games with truly stunning soundtracks:

System Sock 2
Planescape: Torment
The Return of the Obra Dinn

Outcast
Bastion

I enjoy listening to the quantrum sonics radio which is one of the available radios in surviving mars
there's a total of 4 radios are on spotify but I've also seen a list with the whole bunch called surviving mars ost

edit: also dead cells has a great soundtrack

Also adding Furi, Imperium Galactica II and Brigador.

Icewind Dale
Baldurs Gate
Ori and the Blind Forest
Planescape: Torment

I'm surprised no one said Hades yet. Besides that try Bastion, Transistor and Pyre soundtracks.

All the Castlevania and Street Fighter games have pretty good soundtracks. I can also recommend the soundtracks of the Final Fight and Streets of Rage series.

Wow! I gotta say, I never expected so many replies to my thread! Many excellent suggestions of games I've played (and forgotten in my current quest for great soundtracks), and lots of games I haven't played, plus some I've never even heard of. Excited to go through them and expand my soundtrack playlist significantly! Keep the suggestions coming if you have more!

Thanks. Never knew so many of them were on spotify. Time to find them,,,

I don't use Spotify but besides other users' suggestions you could check:

Deus Ex Human Revolution
Alan Wake (some pretty good songs by Poets of the Fall)
Dragon Age: Origins
Knights of the Old Republic
Metro 2033
S.T.A.L.K.E.R. (all of three games)
Myst and all its sequels (Riven, Myst III, Myst IV, probably URU but I've never listened to this one)
Primordia
Machinarium
Oknytt
Neverwinter Nights 2 Complete
Bioshock
Assassin's Creed 2, Brotherhood, Revelations

Deus Ex
Risk of Rain
Hotline Miami 1 & 2

Outside GOG, the Halo games have a good soundtrack.

Inside GOG, I'm very partial to Battlezone 98 and The Red Odyssey.
Battlezone 2 Combat Commander also has a great soundtrack by the great Carey Chico.

Also, XIII (the original, not the remake about which I don't know a thing) has a good jazzy soundtrack.

Slave Zero (Try "135Mix")

That game was worth what I paid for the soundtrack alone :)