I think the best soundtrack of the series belongs to R/B/Y, or Stadium (which mostly is orchestrated R/B/Y music). The restrictions of the GameBoy meant that the composers had to make melodic, simple tunes, instead of the horrible jumble of instruments that is the D/P soundtrack (OK, the Sinnoh Underground/Spear Pillar Battle are good, but the rest is just... I have no word). The R/B/Y music is so catchy and simple, easy to remember and provides tons of neat memories.
G/S/C music didn't stick as much with me. Sure, everybody remembers Goldenrod City, the Bicycle tune, and a battle theme or two, but the music was a little more vague than the R/B/Y one.
R/S/E was good too. Lots of catchy music, and good use of the instruments. Who can forget the blaring trumpets of Route 103? The soft, cozy tune of Fortree City? The awesome that is Battle Frontier? The Oceanic Museum? The PokéMart theme?
I have no high opinion on the D/P music. I think it's all a bunch of "Look how many instruments we can have playing at the same time!!". The battle themes are chaotic and messy, the route themes as generic as the routes they're being played on, and the town themes just forgettable. The Sinnoh Underground music is great, though. Same goes for Route 209 and Dialga/Palkia battle, though in both instances I prefer the SSBB remixes (remice?).
Pokémon Colosseum/XD deserve a mention simply for Pyrite Town's music.
Also, I might risk being banned for advertising here, but this might interest some of the piano players on here (if any):
EDIT: Omicron, we have sheets: http://www.ninsheetm.us/sheets/Pokem...ionCynthia.pdf