Golden Sun and Golden Sun: The Lost Age
These are the best RPGs on the system, as everyone here says, and perhaps the best RPGs of all time on any system. They're not groundbreaking or revolutionary, but they're polished, fun, and pretty flawless
Final Fantasy Tactics Advance
This game has it's flaws, but it's strengths make up for it by far. It's horribly addictive, though. The only other game that forced me to stay up all night is maybe PC Civilization II or III. Not terribly challenging from a tactical standpoint in the way Advance Wars can be (another good game, by the way), this game is more about collecting and developing.
Warioland 4
The big flaw of this game is that it's much much much too short. But that's the ONLY flaw. This has great music, great controls, each level is distinct and yet internally consistent in concept, visual design, and is often terribly clever. It combines slow platform exploration and stressful run-like-hell moments. What a great game. My favorite 2-D platformer, and a game that is all too often overlooked.