I'll give a few GBA titles I love:
Legend of Zelda: Minish Cap, one of the best Zelda games in my opinion, lots of content, great dungeons, and a back story to several key characters and items in the series.
GBA remake of Zelda: Link to the Past. You get to play a slightly updated version of one of the best games on the SNES what is not to like?
Pokemon FireRed/LeafGreen: Remakes of the original Pokemon games, much better than the originals because it has features the third gen games brought to the series. Best of all, a run button! It makes the bicycle practically useless.
Pokemon Ruby/Sapphire/Emerald: The original titles of the third generation. Great region, Pokemon and everything in between.
Golden Sun and Golden Sun: Lost Age: excellent JRPGs on the GBA, interesting story, characters, great combat system, highly recommended.
Mario and Luigi: Super Star Saga: A really strange yet fun Mario RPG. Great dialogue and humor, and a fairly deep engaging battle system.