If you could own only one GBA title, I'd say make it Zelda: Minish Cap. It's so uniformally kick-ass, I just can't figure out how they managed to squeeze so much awesome into one tiny portable game.
What a sweet freakin idea. Take a super kick ass gameplay style like Tetris, add some bitchin' music and cool visual effects, and the results kick ass! Who'd have thought that one of the best reasons to own a PSP...
As busy as my current life style is, I don't have a large amount of time to play all the games that I'd like to. But one title that I somehow manage to keep coming back to for a few quick rounds is Mario Kart DS....