-New Super Mario
-If you like classic platformers with a lot of challengue, pick Yoshi's Island DS. Very good game, and if you want to complete every secret level, you are going to experiencie a great challengue.
-Elite Beat Agents is definitely a must buy.
-Both Nanostray are very good, with amazing graphics.
-But, don't forget, now you have in your hands the system with the most third party and RPG's support. Anything you want: Atlus, Square Enix. Pick any Final Fantasy, The World Ends With You, a Mana series game.
-Phantom Hourglass is very good too: I promise you, you will love Captain Linebeck.
-Mario Kart is awesome too.
-Mario & Luigi Partners in Time is very good if you like RPG's. Very funny game and very well writen.