Well, playable on the DSi, there's just Final Fantasy III and IV.
I've played most of III and only a teeny bit of IV, but generally, people find IV to be a more engrossing game than III, and III has some non-core RPG unfriendly parts (The final dungeon is hours long without any way to save safely, so if you die, you start all the way back at the beginning.) Given this, I'd try IV, since it's also more story-driven, anyways.