Go for Final Fantasy 12 Revenant Wings too.
Might seem slow paced and easy at first, but once you reach the later chapters, you need to really know what you are doing and select spells and targets fast like hell just to survive the enemy mobs, especially at the optional parts.
One of the most challenging RPGs i played on DS, besides FF4 and the Etrian Odisseys.
I'm going to go out on a limb and pick an out there choice: My World My Way.
You get a fourth wall breaking story that deconstructs RPGs, you can literally change the course of the game by pouting, you have a silly putty fighting companion who can mimic your enemies, and lots of magic and such.