The GBA has a lot of great games. Just going by what's on that list, I'd recommend
Final Fantasy Tactics
Final Fantasy IV
Kirby And The Amazing Mirror
Mario VS Donkey Kong
Metroid Zero Mission
Wario Ware
However you should also pick up Zelda: Minish Cap, Metroid Fushion, both Golden Suns, at least one of the Advance Wars games (unless you get Advance Wars DS), Mario & Luigi: Superstar Saga (its DS sequel is really good, too), Castlevania: Aria of Sorrow (again, its DS sequel is a must have), Fire Emblem (and its GBA sequal Sacred Stones, to a lesser extent), and Astro Boy: Omega Factor (Treasure goodness). That's all I can think of off the top of my head, but I'm sure there's more.