The Golden Sun soundtrack. The soundtrack was composed by Motoi Sakuraba, so that should say a lot. From what I remember, it was considered the best video game music for the GBA, and I'm not sure if it's been topped yet, but I love it.
Notable entries:
The First Book (Extended): The extended version of the first game's theme. So epic...
Festival in Izumo: It feels so much like a Japanese festival, complete with taiko drums.
Venus Lighthouse: I think this was considered one of the best songs form the original game. Whether or not that's true or not, I still love it.
Battle! Jenna: Jenna's battle theme. It feels lighthearted, I like it not so much for its battle theme, but its good to listen to whenever I'm doing something.
The bike themefrom Pokemon Gold/Silver. da, da, da, da, dadadadada~