Yeah, this is way too subjective.
I really like the MT-32's sound. It's not always particularly realistic, compared to a Sound Canvas, but it just sounds nice. That's not to say I wouldn't pick a Sound Canvas over an MT-32 for a particular soundtrack, though. Totally depends on the composition.
AdLib vs. CMS is another good example. IMO, the CMS sound is limited in what it can do, and can be a bit aggressive, but I feel like it's a (relatively) timeless sound. OTOH, the primitive OPL2 really shows its age to me, even though it's technically more sophisticated. I would still choose the AdLib soundtrack more often than CMS because a given game's composition was probably optimized for the additional polyphony and range of timbres when both are an option. If you compare NES Mega Man 2 with the Genesis / MD Mega Man port, though, I would take the NES version. It sounds right to me, while the more elaborate FM version sounds like a so-so fan remix.
GUS is more capable than any Sound Blaster (at least up until the AWE) but its compatibility with the PC software library is not so great. So if I had to pick one.... SB16 it is.