Provided you're willing to step out of your comfort zone in terms of genres (indeed, it looks like your current collection is made primarily of RPGs and action-adventure), you're bound to enjoy these. My top picks; I'm not including ports, only original games tailor-made for the GBA.
WarioWare: Twisted! - The best title in the WarioWare series, and the finest title to grace the Game Boy Advance, period. There's absolutely nothing else like it, and the self-contained gyro sensor allows for gameplay that even today's haphazardly developed titles for the iPhone/Touch can't hope to rival.
WarioWare, Inc.: Mega Microgame$ - Twisted's predecessor is still one of the reigning kings of pick up and play gaming. It feels strange having to use the d-pad again after Twisted, but the game is just as cleverly addicting and inventive as its sequel.
Ninja Five-O - A slick and highly polished old-school action platformer. Think Shinobi and Bionic Commando gearing up in a two-piece Elevator Action suit. It's freaken boss.
The Legend of Zelda: The Minish Cap - Not since A Link To The Past has there been a traditional 2D-based Zelda as masterpiece-driven as this. Flagship's done an unbelievable job, and I would have graciously accepted another one like this over Phantom Hourglass and Spirit Tracks.
Drill Dozer - Its gameplay concept is linear, but lengthened to a crafty extent. The self-contained rumble pack adds a tactile level of radness.
Rhythm Tengoku - Arguably the last truly great original GBA title to have been released. An action-rhythm game in its purest form; ridiculously simple but expertly executed, with some of the highest quality music to ever grace the GBA. Wonderful variation in music and scenarios offer lots of playability, and it's a terrific launching point for the even better DS sequel, Rhythm Tengoku Gold.
Kururin Paradise - It's 'Irritating Stick' re-imagined, and a whole lot better. A great pick up and play puzzler that's deceptively more difficult than it looks.
Double Dragon Advance - More remake than port, they really gave the original Double Dragon a serious makeover. All new levels and enemies, rearranged music, and a gaggle of new fighting techniques. The GBA's premiere beat-em-up.
Advance Wars - Still one of Nintendo's best turn-based strategy war games. Its simple anime presentation belies the deep, diverse, and challenging gameplay. If you become invested in the first game, check out the sequel, "Black Hole Rising."
Mario vs Donkey Kong - Controlling Mario feels wonderful; Nintendo and NST did a great job at gathering the plumber's repertoire of moves throughout the years, and condensing them into a really tight package of puzzle and platforming.
Mario & Luigi Superstar Saga - Though last year's Bowser's Inside Story dethroned this as the best of the M&L RPG series, Superstar gets mad props for delivering a robust quest, sharp, hilarious writing, and great battle gameplay.
