Again I differentiate tastes by category, since not everybody likes the same stuff.
Pokefans mostly, but they're respectable RPGs nonetheless: Pokemon RPG series - Emerald most of all
Best RPGs for GBA: Golden Sun series - The Lost Age but ties with the first game so you should play both
Something if you like an action/RPG mix: Castlevania - Harmony of Dissonance and Aria of Sorrow
Shooter fans: Doom 1 and 2, especially 2, and Duke Nukem Advance
Strategy/RPG: Final Fantasy Tactics Advance(I honestly didn't like Advance Wars, despite what you all may say)
Adventure/RPG: Zelda - Minish Cap
I have to say if you want one that's truly impressive go for Golden Sun 2(The Lost Age): fair story, excellent gameplay, spectacular effects, great music and LOOOOONGASS game with a huge map that's bound to keep you busy for many many hours. I totalled 52 hours to get everything done and level up decently. Playing the first game beforehand is highly recommended. You get to transfer your characters into 2 and access endgame content. GS1 is the same as 2 in every positive aspect except it's shorter but has identical graphics and such.
