Subarashiki Kono Sekai is probably the most unique RPG I've ever played on the DS.

Mario and Luigi: Partners in Time

my definition of RPG has always been a gameplay that depends on your stats, strategy and luck rather than skill to affect the outcome of the game. Traditional RPG would one way or another implement the dice roll mechanics (1d6, 2d20 lol) that's similar to D&D tabletop RPG. The fact that it's turn based (think FF), or combat/auto dice roll based is just a matter presentation as the basic mechanics still apply.

So Zelda games are what I would call Adventure games, rather than traditional RPGs.
My favourite RPG for the DS would be Revenant Wings. But it's more of a Strategy-RPG than anything else.
also Pokemon, obviously.

I'm looking forward to The World Ends With You, and also that Sonic RPG produced by Bioware.