IMO:

Phoenix Wright: Ace Attorney
Viewtiful Joe Double Trouble (this is even better than the console versions, and actually uses the touch screen to the fullest, unlike other DS titles)

And if you can import games, then get Rockman ZX, especially if you like Castlevania/Metroid-like level structure. If not, then wait for the English version that comes out next month.

Other than those games, Mario Kart DS, New Super Mario Bros., and Sonic Rush are pretty damn good.