I pretty much hate the music after Gen 2. Which have low sound quality for obvious reasons, yet somehow the remake versions of the same songs are even worse, because they rearranged all the songs terribly.
Hmm...got to go with gen 5 here. All of them had some nice music. The first one was very iconic of course, the second one was very solid as well. Gen 3 was musically good, as in, it had some good tracks, but I didn't like the instruments. I like Gen 4's music more than most others here it seems, I really like some of the route tracks, Cynthia's pre-battle (the piano stuff) and battle music and I thought the Game Corner was pretty epic. :p Oh and the pixie battle music was great as well.
But I vote for gen 5. Possibly the best collection of town music (Driftveil, Accumula, Nimbasa, Opelucid...), best battle themes, Alder's theme (when you encounter him during the storyline), B/W's route 10, legendary theme...good stuff.