I played the hell out of my C64 back in the day as a kid and I can definitely attest that some of the absolutely shittiest games for it still had kickin' soundtracks.
Probably one of the best known, stand-out composers of the C64 era was Rob Hubbard and I'm pretty sure he has some collections out there.
My big gripe with Hubbard though is that, to this day, he claims to have done all the music for the C64 port of Capcom's "Commando" when in fact his music was remixes of the original soundtrack which was actually done by a young Tamayo Kawamoto before she became part of Sega's SST Band (and later, Zuntata)