Pretty much every MM game I've played had a great soundtrack, if I had to pick a favorite... for the original it's MM2, 3, 8, 9. The X series generally had better music (and was a better series overall for me) and X4/X5 probably had my favorite music in the entire series. Network Transmission from the Battle Network series also had a really good soundtrack
Favorite classic game is MM3 with MM5 and MM8 right behind it, for some reason I'm not as big on MM2 as others are even though it was the one that got me into the series, it probably has my least favorite Wiley levels (music kicks ass though). Favorite X game is probably X4 but X1-X5 are pretty close for me