1. Mario & Luigi: Superstar Saga
I can't say enough good things about this game. It has a genuinely great sense of humor that so few games can pull off. It's got so many refreshing gameplay systems. I usually dread turn-based RPG combat, but this game turns all that on its head. You can actively dodge, counterattck, and bonus attack during battles. Out in the world, theres lots of platforming and different gimmicks to be found. This game is fun distilled into a perfect little cartridge. This is a game that proudly and happily announces that it is in fact purely a videogame.
2. Advance Wars
Hell of a game here. It's got a relatively simple system of seemingly standard strategy (I should totally make Nintendo buy that as a slogan) game mechanics with some very deep and satisfying gameplay. Countless hours will be spent with this game, and it'll even be in your head when, for whatever reason, you decide to not play it (you must be crazy). It's got a very "rock, paper, scissors" sort of feel to it, but imagine they added in "stapler, hammer, torch" into the mix and made it fun (and much less painful and scarring).
3. Castlevania: Aria of Sorrow
Out of the 3 Castlevania releases on the GBA, this one is the best. That's not an easy statement to make, but only because they're all such great games. Harmony of Dissonance and Circle of the Moon deserve more than an honorable mention here. Aria of Sorrow takes the now familiar post-Symphony of the Night (Metroidvania) formula and makes some great additions. The ability to steal the souls of your enemies being chief among them. It's a lengthy adventure with a story that's got some great twists in it. Plus, it's Castlevania on the go. Pretty sweet deal.
4. WarioWare, Inc.: Mega Microgame$!
Stay focused, because you have no idea what's gonna happen in the next 2 seconds; such is the thrill of WarioWare. Whether fighting off Mech Bowser with a QTE as Mecha Mario, guiding link into a cave, threading a needle, dodging a salvo of arrows in medieval Japan, or simply catching a piece of toast- you are constantly being bombarded with fun. And why is this all happening, you ask? Wario needs to get that green, son. And what better way to do it than with a get rich quick scheme... that he makes his friends do for him. What a bastard. Seriously.
5. Golden Sun
A perfect infusion of everything you loved about SNES era RPGs in one surprisingly awesome and original game. This one is damn near a masterwork of elegance and simplicity. While it does seem to stir some nostalgic echoes, GS by and large does it own thing. An intertwined monster-collection/summoning/stat boosting mechanic makes the turn based combat a lot more nuanced and interesting than other games of its ilk. Pair that with a coming-of-age story that deals with themes of personal loss and courage in the face of impossible odds and you've got an instant GBA classic that still holds up well to this day.
6. Final Fantasy VI
This game is another heart-melter for me. One of the first JRPGs that did the whole "amnesiac protagonist" thing in a way that didn't make me roll my eyes, FFVI is widely considered one of the best games of all time- and the damn thing still holds up today, and as a portable game, nonetheless. It's got some of the most endearing and memorable characters you'll ever play as and with beautiful music to match. Even in 2009, the Opera House scene still struck me as moving and wholly original. It's a tough find, but it's worth it.
7. Wario Land 4
Wario is kind of a bastard, and I love him for it. So when a pyramid emerges in the middle of the desert, and this guy sets out to plunder it, could I resist joining him? Nope. It's a great platformer with an even better sense of humor that permeates everything from the sound to the gameplay to the graphics. And if for some reason you don't quite feel like adventuring, you've always got a few choice psychotic mini games to play with.
8. Yoshi's Island: Super Mario Advance 3
Take a look at this game and I bet all the coins in the kingdom your cold heart will melt. This game is just so.... happy. It's pastel visuals convey a sense of joy that no other game even comes close to delivering. Add to that some rather unique "shooting" elements, spot on platforming, and an endearing play on your paternal instincts and you've got a game that tugs on your emotion box as it begs to be played. Awwwww.
9. Street Fighter Alpha 3
It's Street Fighter. But Alpha. And the third one. Also, it can fit in your pocket. If you need anything else other than that, you are spoiled beyond belief and nothing can really ever satisfy you. A huge roster, a monstrous move set, made even more monstrous because of 3 different fighting styles, fluid, vibrant, flashy graphics, and the classically simple yet deep gameplay of Street Fighter all in one tiny GBA cart... What else should I put down in this blurb? Nothing. Go away.
10. WarioWare Twisted!
So this a game that again surprised me. Again, I have underestimated just how clever the warioware team is. Tilting? How could that be fun? So what if it's got vibration feedback? And so for 10 bucks, I though "eh what the hell, the other one is good" and... I haven't played anything else since I got it. It provides such a great tactile experience and still with such a great sense of humor. Pounding nails in, moving giant legs to avoid stepping on little people, flying Wario to a treasure- all handled marvelously with tilt. And it works better in this little cart than it does on any console. How the hell do they do it? I don't know. Maybe they don't either. Just go buy this.
11. Metroid Fusion
This game gets a nod for being one of my favorite games of all time. It's the 1 only because I could never get into the "portable" mindset when I was playing it. Also because you really don't have a quick save option. Save stations only. This game would absolutely give me the outward appearance of a zombie. With some of the absolute tightest gameplay, prettiest visuals, and sound that totally immerse me into the experience in unsafe amounts- Metroid Fusion is the must own, must play game on the GBA. I replay this game once a year and it's just as awesome every time. I'm being somewhat cryptic because I believe it needs to be experienced. Go pick it up cheap, you won't be sorry.