The Roland SC-55/SCC-1 one would probably be an authentic sounding choice for GM/GS game soundtracks after the early 90s, since a lot of the soundtracks were actually composed with it. Doom (and its sequel), Duke Nukem 3D, Descent, the Warcraft games (though the WC2 composer(s?) used an SC-88 as well) just off the top of my head.
Or Masquerade 55, if you want higher fidelity but with SC-55 replacement in mind.
Space Quest III predates General MIDI altogether. I assume that the original soundtrack was composed for the Roland MT-32 and that the ones you are listening to have been remade into GM by fans, or maybe it was added in a later version. It would be nice if this player contained an MT-32 emulator since so many Sierra (and other) game soundtracks were composed for it specifically. Such a thing already exists: https://github.com/munt/munt
GMGSx Plus is basically a Roland SC-55 sound-alike, except with the Piano patch swapped with a nicer one from GeneralUser GS. Good tradeoff of size and sound quality.
There's a huge variance in Soundfont quality, and the craft put in by Soundfont authors to program all the modulators properly makes a huge difference. GeneralUser is one of the best. https://schristiancollins.com/generaluser.php
This is wild. Anyone who’s more into this space than I am have any editorial comments on the various SoundFont options?
…listening to Space Quest III…