I used to love the music that played when you entered the sun bank. Good times. Some of the dungeons had pretty atmospheric tunes too.
There's lot of great stuff on the GBA that's gone largely underappreciated, probably because GBA music gets a bad rep from some of the ports that tried to replicate the SNES's sound chip poorly.
The Bit Generations games have
some excellent music, though I get most people probably haven't even played them!
Drill Dozer has some top notch music that
makes excellent use of both the GBA hardware and GBC's sound generator.
Same goes for Konami Krazy Racers. Anyone remember
Sky Bridge?