From what I've played? From best to worst:
GSC
RSE
DPPt
RBY
GSC's music was overall the best put together and even though it was 8bit, you could still visualize what instruments they were trying to simulate.
RSE's really good music was rare, but it was still there, like the Elite 4 music.
DPPt's music was irritating, childish, and a lot of it felt like music that was ripped from GSC.
RBY's music was just horrible. The champion theme was a random string of notes. Lavender town's theme was so out of harmony that it was rumored at one point to cause cancer or something.