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.