Since I like the small collection of DS games I own, I'd recommend all of them:
Mario Kart DS (still, imo, the best Mario Kart installment to date)
New Super Mario Bros.
Castlevania: Dawn of Sorrow
Castlevania: Portrait of Ruin
Final Fantasy III
Legend of Zelda: Phantom Hourglass
Tetris DS (sounds boring, but it is mind-bogglingly addictive)
Metroid Prime: Hunters (Good if you want a multiplayer FPS on the go)
I've also heard that "The World Ends with You" is quite good. Also, Final Fantasy IV is on the horizon, and is definitely, imo, worth picking up if you don't have a copy of FF4 in your possession or if you want to enjoy FF4 remade in 3D (like FF3 on the DS).