From my own collection:

Magical Starsign - a nice, silly JRPG with DS interface.
Etrian Odyssey - very difficult old school PC style RPG based around exploring and mapping dungeons. Love it.
Freshly picked: Tingle’s rosy rupee land - not played it yet but it's so wierd it has to be good :p
Dragon Quest Heroes: Rocket Slime - even better if you've played any Dragon Quest games and can catch the in-jokes.
Pokémon Pearl - just because.
Castlevania: Dawn of sorrow - another good entry into Metroid style side of the series
Castlevania: Portrait of Ruin - ditto above
Kirby: Canvas Curse - inventive use of stylus control for platforming.
Mario Vs Donkey Kong 2: March of the Minis - it's Lemmings but better
New Super Mario Bros. - if you like platformers
Yoshi’s Island DS - ditto above. A much more difficult game despite the cute packaging
Phoenix Wright: Ace Attorney - think point and click reborn in an insane courtroom, right down to the funny dialogue a la Moneky Island
Phoenix Wright: Justice for All - slightly inferior sequel to the above. Play them in order for plot purposes.
Hotel Dusk: Room 215 - more point and click-esque goodness.
Meteos - puzzle game but not boring like Tetris :p
Picross DS - another puzzler.
Trauma Centre: Under the Knife - difficult. Inventive. Difficult!
Age of Empires DS - I found it better/less annoying than advance wars DS.
Anno 1701 - outstanding port of the recent(!) PC strategy game