usually platformers and sometimes rpgs and racing games
Platformers?
Super Mario Advance series have good replay value. Just patch them to correct colors and add e-Reader content and you're good to go.
Megaman Zero is challenging if you like playing platformers that will cramp your hands.
WarioLand 4 is ok.
Kirby games are below average in my opinion. You're really not missing much.
Metal Slug Advance is ok.
AstroBoy is good at first, but gets monotonous quickly.
Batman Begins is ok. Might kill 5 minutes.
Spider-Man games are ok. Never been a fan of licensed games outside of SNES era.
I don't like Sonic, so I don't know about the Advance series.
Gunstar Heroes is good.
Metroid Fusion is the best. Zero Mission is ok.
Castlevania Aria of Sorrow is the best. Circle of the Moon and Harmony of Dissonance aren't quite there on par with Aria of Sorrow.
Mario & Luigi Superstar Saga is really good. You should definitely play that one. Good mix between RPG, Puzzle, Platforming and Action.
I wouldn't recommend DKC Advance games. They just... exists.
Ninja Five-O offers an arcade-like experience. It's good if you like that kind of games. Not the best platformer, but definitely better than Kirby or DKC.
Super Ghouls'n'Ghost exists as a stress test. I like the SNES and Arcade games, so of course I like it. But might be too difficult for "gamers" that aren't as old as me.
Dragon Ball Advanced Adventure is really good. Really nostalgic and the kind of game you'd expect from a Dragon Ball licensed game.
As far as racing gamed go... I don't like the GBA limitations when it comes to these kind of games. A lot of the experience is lost thanks to the graphics and controls. But I can only recommend a few.
Konami Crazy Racers is a Mario Kart but with better controls. Catchy music and fun gameplay. Definitely check it out.
Mario Kart Super Circuit is an obvious choice. It takes time to master it's slippery-ness but once you've got the hang of it, it becomes a feature that you can overuse in some tracks to make quick or difficult turns. Also, with enough experience, you can literally go so fast that Red Shells can't reach you, on top of that, you too can jump, so Red Shells pass you by. The "Retro Tracks" is good unlockable content. One of the best MK games once you master the gameplay mechanics.
Forgot to mention, you can throw Red Shells behind and they'll literally chase anyone behind you.
SSX3 is very ambitious. Sure it's very, VEEEEEEEERY watered down, but it excels at gameplay and content. Again, once you master it's controls, it gets very fun to replay thanks to the "Character Customization" feature.
Edit: Silly me forgot F-Zero. Definitely this one.
Hot Wheels Stunt Track Challenge and Hot Wheels World Race are ok.
With RPGs... there's only one "genre" I like, and that's the A-RPG.
So Sword of Mana and Zelda are must plays.
As far as traditional RPGs goes, Final Fantasy VI and Golden Sun are the best.
Lady Sia is a very early GBA looking and feel game, but it's entertaining.
Pokemon, of course. Emerald is the best one.