Descending order, * = must-buy:
-Pokemon Fire Red/Leaf Green - Awesome, awesome, but will be rendered obsolete to some degree by D/P. Becomes a must-buy in the context of new entrants to the series, though, since the original quest, music, and Pokemon are all still the best.
-Pokemon Emerald* - Though the "main quest" isn't as good as FR/LG's, this gets must-buy billing because the Battle Frontier gives it longevity that might just last even after D/P.
-Legend of Zelda: A Link to the Past - Still the best Zelda IMO; the balance of exploration and dungeons, the balance of fighting and puzzles, the music, and the diversity of the item set are what make it. Doesn't get the *, though, because it's not a must-buy unless you don't own it on SNES, or happen to know other people who bought this GBA game and own link cable(s) and will play multiplayer with you.
-Yoshi's Island* - Outperforms its SNES version thanks to six more levels, which is saying a lot since YI is the best platformer ever made.
-Advance Wars 2* - In spite of AWDS being out, AW2 continues holding two advantages over its sequel, one small and one big: a non-isometric view, and a better campaign.
-Mario Tennis Power Tour* - Enormously underrated. I didn't even like tennis until I played this, but MTPT is one of those rare games that's successful on every level of its gameplay and in every aspect of its production.
-Fire Emblem* - It's Fire Emblem.
-Kirby and the Amazing Mirror - Would get a * if Kirby Squeak Squad wasn't coming and wasn't probably going to be even better.
-Metroid Zero Mission - Very close to a must-buy, but falls short on the basis that its length balance is off--it's too short to play at home since you can beat it in one sitting, but too long to play as a real portable game unless you fly a lot, or commute for two hours on a train or bus.
-Fire Emblem: The Sacred Stones - A notch below its predecessor due to the music, characters, and story, but still great. If you can't find FE7, nothing wrong with this at all.
-Tactics Ogre: The Knight of Lodis - A great strategy game, though not a necessity since FFT (PS1) can fill that role moderately enough--and it's not like you'll ever find a copy of this anyway. But hey, Luminous Arc is coming on DS, right?
-Castlevania: Circle of the Moon - Very good gameplay, though irrelevant in the face of Dawn of Sorrow and (probably) Portrait of Ruin.
Edit: My comments are assuming that most people who would click on a GBA thread in a hardcore Internet gaming forum own a DS. If not, then sure, bump Castlevania and Kirby into the must-have category.
