I'll start. :p

-The Legend of Zelda: The Minish Cap
Without a doubt, my favorite 2D Zelda game, and probably my favorite Zelda game overall. It has a ton of content outside the main quest (Kinstones, minigames, etc.) and I even found collecting the little figurines strangely addictive. Also, it has some of the best graphics and sound on the GBA.

-Riviera: The Promised Land
While this game garnered mixed reviews from the gaming press, I found it to be a lot of fun. While the exploration and combat are different from any other RPG game, I found it to be a rewarding experience. Great music, sound, and story make this one a joy to play.

-Final Fantasy Tactics Advance
Tactics games have always been one of my favorite genres, and this game is probably the genre's pinnacle. Not only does it have the great sound, story, and art style you would expect from a Final Fantasy game, the tactical combat is deep, while still easy to control. The main story lasts about a dozen hours, but the side quests and extra content can keep you playing for hundreds of hours.

-Pokemon: FireRed/LeafGreen
While the Pokemon formula might not be for everyone, you probably have played one or two of these addicting RPGs. What are essentially remakes of the first Pokemon games, FR and LG add a ton of polish and extra gameplay. The graphics, sound, and interface are much improved, and the core gameplay is still as addictive as ever. Aslo, after you complete the main story, you open up access to a few more islands to add many more hours of gameplay.

-Mario & Luigi: Superstar Saga
Taking the core gameplay from Super Mario RPG (snes) and adding an extreme layer of polish and humor, Mario & Luigi: Superstar Saga is one of the best, if not the best RPGs on the GBA. The story and world are completely unique and engaging, and the fun minigame-style combat never gets boring and keeps you on your toes all the time. A must play for Mario fans and RPG fans alike.