Get Drill Dozer. It's a hugely entertaining platformer which might remind you of some hot-blooded anime with drill mechas.

It's also decently hard, and has a rumbling cartridge!

Astro Boy: Omega Factor, both Breath of Fire ports, FFIV and FFVI ports, FF Tactics Advance, Castlevania Aria of Sorrow, Disney's Mystical Quest 3, Mega Man Zero (any of them), Mother 3(just kidding), Yoshi's Island remake and Seiken Densetsu remake (I think they named it Sword of Mana).

Just look at the GBA games I have, I would whole heartedly recommend every single one of them

Thanks for the input guys

I just borrowed Metroid : Zero Mission from my cousin...its kinda confusing at first, and I believe I finished the first level ! Its pretty fun

Holy shit almost forgot: Kingdom Hearts: Chain of Memories.

If you enjoy the Metroid Games (as did I! ) I think you'll be a fan of the Castlevania games as well on the handheld. There's three of 'em: Circle of the Moon, Harmony of Dissonance, and Aria of Sorrow.

Circle of the Moon was my first ever Castlevania game, and it was fantastic. It made me into the Castlevania fan I am today.

Never played Harmony of Dissonance, but I'm always on the lookout for it. I heard it was the weakest of the three GBA games, but still a solid game.

I just found Aria of Sorrow a couple of months ago and bought it right away, and It was money well spent. It's a rare Castlevania in that it's part of a direct line (Castlevania: Dawn of Sorrow on the DS), and it's really awesome. Up there with the best I've played on the GBA.

Golden Sun, Castlevania Aria of Sorrow, Zelda A Link to the past plus four swords, Advance wars, Warioware twisted, and Yoshis Island

Metroid

Metroid: Zero Mission

Metroid Prime

Metroid Prime: Hunters

Metroid Prime 2: Echoes

Metroid Prime 3: Corruption

Metroid II: Return Of Samus

Super Metroid

Metroid: Other M

Metroid Fusion

Thanks dude !

The Mega Man Zero games are pretty good.