Metroid Zero Mission is great. Metroid Fusion isn't so great, but still fun.
Wario Ware is probably the best GBA game ever, though some people absolutely hate it. It seems to be a black and white thing though: either you love it madly or you despise it.
The Super Mario collections are pretty fun. Super Mario World, Super Mario 3, etc. Good for that nice retro feel.
The Legend of Zelda: A Link to the Past kept me busy for quite a while. Definitely on the list.
And you can't go wrong with some of the old-school Gameboy games. The Super Mario Land series for example.
So damn many games for GBA that are good, especially with downward compatibility.