I have got a load of RPGs for DS, and I'd probably recommend all of them really.
Top of the list is The World Ends With You. It is a fantastic Action-RPG game. It is unique, fun and has a ton of replay value.
Unfortunately I don't have Dragon Quest IX, but I would certainly recommend DQV. It has a great story, and is really fun to play. I am a fan of the DQ games, and if you are too, definitely get it.
One more I will recommend is Children Of Mana. It's made by Square-Enix, so you know it's going to be good. It plays like Zelda really, especially the DS Zelda games. It is a really fun game, with plenty to do. It's also a good challenge, so if you like to be challenged this is for you.
Other than that, the FF games are great, the Zelda games are good. Obviously, Pokémon is a must have for DS, and also Kingdom Hearts 358/2 Days.