Ghost Trick - Phantom Detective. If you like Phoenix Wright or Professor Layton, this is a game you will enjoy.
Sadly, you will only be able to play it once. I can't say anything else without spoilers, but it is an exceptional game that you will never forget, and singular in its brilliance in storytelling. You will NOT anticipate the story, which is unheard of even in films and literature.
The World Ends With You. This is probably one of the greatest action RPGs of all time, on any platform, but especially on the DS. A lot of shovelware came out for that console, mostly trying to find new ways to use all of the DS hardware like the microphone and the touch screen, to mixed results. THIS game gets it perfectly right. Granted, the online stuff is now borked, but there are workarounds. It still uses literally every single feature of the NDS without it being bullshit like in Castlevania - Dawn of Sorrow, or holding a button so you can scream OBJECTION! It is one of the best RPGs ever made, and I think the DS version is still the best possible way to play it.
And that said, Castlevania - Dawn of Sorrow. If you like platformers, and you like Metroidvania, this is the best title on the console, and the closest competitor to Symphony of the Night to date. Massive map. Tons of collectibles. Lots of customization. Pretty decent story, if you care. This came the closest to the experience I had when I played SotN for the first time. It's amazing what they packed into this game, and how well it still holds up today.