Mario & Luigi Superstar Saga
Swordcraft Story 1 + 2 (it's basically a playable manga, get it! and pick the lesbo in part 1 and the tsundere girl in part 2, best dialogues ever in any rpg!)
Mario Bros 3 and Super Mario World remakes
Zelda: A Link to the Past remake
Zelda: Minish Cap
Golden Sun 1 + 2
Advance Wars 1 + 2
FF Tactics Advance (but the battery in my module went out of juice and I can no longer save)
Metroid Zero Mission
Pokemon 3rd Gen (Ruby Sapph Emerald Firered Leafgreen)
Donkey Kong Country 1 - 3 remakes

not sure if actually liked the GBA

but it does have the best castlevania game ever made
Castlevania Concerto of Midnight Sun
and the second best castlevania game ever made
Castlevania Minuet of Dawn

I don't really have a favorite, but Crash Bandicoot, Pokemon, and Castlevania.
There was another game I often played, but I forgot what the name was. If I remember correctly, you play as a blonde girl with army clothes (or something like that)...I'm pretty sure the name had something along "portal" or "combat" in it.

Pokemon =p, it's so childish yet so addictive

Golden Sun 1 + 2 ^^

I'm gonna have to go for the Pokemon games. Easily the only game that I have ever wasted so many hours of my life on.

the Golden Sun games
the Final Fantasy games
Tales of Phantasia

Riviera the Promised Lands hands down.

We all suffer the same pains and we all feel the same pleasures.

all final fantasy and pokemon gba games.

pokemon gold and silver
lord of the rings: the two tower and
mario go kart

Harvest Moon (All. I think there are 2-3 versions.)
Pokemon (All.)

Does the walker choose the path, or the path the walker?

Pokemon FireRed/LeafGreen - I'm not really into these games as much as I used to be and I didn't like Ruby/Sapphire, but Nintendo knows how to exploit my nostalgia. I almost cried when I entered Pallet Town and heard the music.
Golden Sun series - Didn't age too well, but they are some of the best RPGs on the GBA, and I still have a soft spot.
Super Robot Taisen J - Because you have to play one on every platform.
Final Fantasy Tactics Advance - I put 100s of hours into this.
Castlevania: Circle of the Moon - my introduction to the series.
Mario Power Tennis - Golden Sun + Mario Tennis. Or rather, what Prince of Tennis would be like if it were made into an actually good game. Fucking awesome, and also nuts.
Rockman.EXE - Interesting new interpretation that I fell in love with. Suffered from sequelitis, though.

I always wanted to try Advance Wars, but I never did

Quote from corx_d_afikolami

I did actually played quite a lot of GBA games and these are the ones that I got really addicted to:

Both Golden Sun games
Riviera the Promised Lands
Yggdra Union We'll Never Fight Alone
All the Final Fantasy remakes; I, II, IV, V, VI + FF Tactics Advance
Both Summon Knights games
Kingdom Hearts Chain of Memories

its exactly as my favorites so i just copied it 😀 right now playing golden sun dark dawn and have to say its worth its prequels 😉
oh and i forgot about one game: the legend of zelda minish cap 😃 really enjoyable game

Minish Cap <3
Castlevania: Aria of Sorrow
Fire Emblem 6+7, 8 not as much
Summon night (Haven't played 2 yet)
2nd gen Pokémons (played: Gold and Crystal, haven't played: 1st gen to the end, 3rd gen+ at all)
Tales of Phantasia

I still haven't played: Advanced Wars, Golden Sun series, Mother 3, any of the Final Fantasy games, Harvest Moon...probs more that I can't remem atm

Final Fantasy tactics
Legend of Zelda
and all Pokemon GBA games are my favorite ever!

One Piece Episodes