I´ve never been a huge fan of the DS, so I decided to skip buying it and instead sticking with my GBA. Although games are getting harder and harder to find, they are also becoming cheaper, reason why I believe it is a good idea to build a solid collection of GBA titles. This thread is to recommend the best of them, of any genre (I am personally trying to complete my collection, so any help would be appreciated).

Final Fantasy I & II Dawn of Souls

"Golden Sun

Golden Sun: The Lost Age

Advance Wars

Advance Wars 2"

Golden Sun, Golden Sun: Lost Age, Metroid Fusion, Fire Emblem and Fire Emblem: Sacred Stones, Pokemon Firered and Leafgreen if your into the Pokemon, Final Fantasy Tactics Advance, and any of the Advanced Wars games

Hope that helps,
Nails

pretty simple ...

all castlevania titles
all metroid titles
all zelda titles

when you are finished ... move on to the same games on their DS releases...

I only have two games:

Super Mario Advance: Super Mario Bros 3
Metroid Fusion

both awesome

Dude, I can't believe no one has mentioned WarioWare: Micro Minigames. It's probably the game I played most on GBA. If you've played any of the WarioWare games, you'll know what to expect, except this one is the original. If you haven't, prepare yourself for a totally crazy, fun game. WarioWare: Twisted was the follow up with a built-in tilt sensor, and while it's also cool, I still think the original is better.

• Golden Sun *MUST BUY*
• Golden Sun: The Lost Age *MUST BUY*
• ... Pokemon Ruby/Sapphire... I had loads of fun with them, logged well over 100 hours of playtime on Ruby alone... damned Feebass.

The WarioWare games are solid. Try to look for a copy of AstroBoy.

Some great suggestions so far, the only one I see missing is Mario & Luigi: Superstar Saga

There have been a lot of suggestions on games that I would say, so instead I'm going to give a shout out to the other reason I keep my GBA SP healthy, Game Boy and Game Boy Color games.  I love playing some of my originals (yes, I'm old) games, including Super Mario Land, Pokemon Puzzle League (much better than Planet Puzzle League), and Metal Gear Solid.

I have to say some amazing suggestions so far, can not go wrong with a single title listed thus far into the thread...

Oh and you need to try out all the Bit Generations games, you can only import them but they are fully in english,  and you need to try R Tengoku quite a new experience 

All Castlevania (especially Aria of Sorrow) and Metroid games are great on GBA. Advance Wars 1&2 are great too. There are also a lot of sweet rpgs for this system. :3

Although not actually a gba game, you can get the Phantasy Star 1,2,3 collection from play or ebay relatively cheap and they are faithful recreations of the originals.  If youre into that kinda game, its well worth a look.

To add to the fine games others have already mentioned.

Mario Golf
FZERO Maximum Velocity
Wario Ware Twisted
Castlevania :Castle of the Moon 
R Tengoku.
Konami Kart Racers
Astroboy
Gunstar Future Heroes
LOZ:LttP
Super Mario Advance 3: Yoshi's Island
Mario Kart: SC
Sonic Advance 2

...and 2 copies of Street Fighter Alpha 3 and a link cable.

" There have been a lot of suggestions on games that I would say, so instead I'm going to give a shout out to the other reason I keep my GBA SP healthy, Game Boy and Game Boy Color games.  I love playing some of my originals (yes, I'm old) games, including Super Mario Land, Pokemon Puzzle League (much better than Planet Puzzle League), and Metal Gear Solid. "

Golden Sun.

It's been suggested a few times already, Mr bandwagon rider :)

After browsing through my collection..

Wario Ware
Super Mario Advance: Super Mario World
Advance Wars 2 (!)
Sonic Advance 1
Tony Hawk Pro Skater 2
Puyo Pop

I liked Metroid: Zero Mission more than Fusion personally.