I really like the music in Pokemon Fire Red/Leaf Green because it is both 32-bit sounding and 8-bit sounding at the same time.
^This. I love this game's music.
Edit : Here, have some eargasm.
It's awesome how much more detailed it sounds on my mid-end stereo than on the GameBoy itself. Guess they made it too detailed. I noticed that it also happens on the DS with internal speakers vs earphones.
I also love SMB DX music and all the Zelda themes.
Phoenix Wright on the DS has some particularly epic music too.