What do you mean by RPG with the best music? I'll just answer both possibilities.
RPG series with the best music: Dragon Quest
RPG Game with the best music: Tough to decide between Chrono Cross, Dragon Quest IV, Dragon Quest VIII and Final Fantasy VI. All of them have amazing music both in game and while listening to them alone. There's no song you want to skip and some areas you even want to revisit just to hear the song.
Some good examples.
Final Fantasy VI - The Phantom Forest
Dragon Quest IV - Comrades
Dragon Quest VIII - Mysterious Tower
Chrono Cross - Another Guldov











