I guess it depends on what you like.
If you enjoy text adventure/visual novels, there’s a bit of good stuff on the DS, like Trace Memory or the Ace Attorney games.
If you enjoy the Castlevania series, I reccomend Dawn of Sorrow and Portrait of Ruin. I guess the newest one isn’t terrible, all things considered, but it takes a lot of steps backward in the innovation department.
If you enjoy tactics RPGs or turn-based strategy, I DEFINITELY recommend Advance Wars, especially the newer one, Days of Ruin.
Strangely, the place that I find the DS to be lacking in severely is RPGs. Children of Mana starts out really fun, but it gets old really fast (which is surprising, because the game is very short, like maybe 12 hours long).
The Lunar RPG on DS was a disaster, as are quite a few other ‘big name RPGs’ like the FFTAdvance game, Sonic Chronicles (though maybe you expected that), Final Fantasy 3 DS, Front Mission, and Blue Dragon Plus. I didn’t like Final Fantasy 4, but I remember back in the days that you enjoyed games with difficulty levels leaning on the side of absurd, so maybe you’ll like it. 
Dragon Quest 4 is good, but it’s not technically an original game. Still, if you enjoy the Dragon Warrior games, DQ4 is the hotness.
If you wanna know what my favorite RPGs on the DS are, it would definitely be The World Ends With You, and My World, May Way, although My World, My Way is definitely not for everyone; it’s a satire on several shitty gameplay aspects of RPGs, but the irony of course is that, in satirizing such things, the game IS a bad RPG. Still, if you appreciate the satire, the game is hilarious. As for TWEWY, well, I’m sure a ton of other people will come in and tell you how great that game was, and if they don’t, I’ll come back and tell you some shit. 