I you haven't played the Hotel Dusk or it's sequel, you simply must do so. Simply amazing.
If you run out I'd suggest checking /v/'s Recommended Games wiki. It's a great place to get a quick idea of what games are worth playing including a lot that aren't quite mainstream.
I think pretty much everything mentioned in this thread is on that list, though I can understand wanting a personal touch in your answers that suits your style of gaming.
Oh, and if you do play The Last Window (Hotel Dusk sequel), you'll have to do it the shady way, but no game is more worth it than that one.
Honestly, it's gotten so hard to find some DS games, especially the slightly more niche ones I haven't played yet, that I've fully given in to the dark side and almost exclusively emulate because I don't want to pay $80 for only the cartridge with the label half scratched off.