I think everyone agrees here that the GBA is where you should go for RPGs. Golden Sun 1 & 2 own, and so do the old FFs. When the DS first came out, I hardly got any games, because I was waiting for some good RPGs. I tried Magical Starsign, but it's far too basic for a good RPG, and generally was very average.
So now I've had to make do without RPGs for a while. I generally like to play Phoenix Wright now anyway. There's quite a lot of old PC RPGs you can find on the internet if you're desperate, KOTOR is really good, and so's Oblivion if your PC can run it.
EDIT:
Just remembered there's always Children of Mana, which is good, but a bit repetitive.
Plus, there are some Final Fantasy and Dragon Quest games heading towards the DS, so watch out for them.