kinda hard to choose ngl, because i feel like you'll find discussion of almost all of the massive amount of great games on the GBA, from sonic advance (though its rather controversial...) to almost all of its RPGs, etc.
One I see very little discussion of is Phantasy Star Collection. I love the original PS games, but sadly I don't see much discussion of them in general. And well, they're on the GBA, and thus are some of the best RPGs available on the system.
There are also primitive Celeste games available. There's the original pico-8 classic Celeste, but also a fan made Celeste 2. TMK someone's trying to port the full game to the GBA too, though that'll certainly take a bit and won't really match the original in form. I played classic, it's similar formula and thus a good time.
There's also fullmetal alchemist RPGs but I've not played'em yet so no comment.
