1. Mario Kart DS - The definitive Mario Kart game, lots of great levels, solid online system. Only beaten by Mario Kart 64, which I still consider the standard by which Mario Kart games should be measured.
2. NSMB - Not the greatest 2-D Mario platformer, but hell, it's still a 2-D Mario platformer, and one that was long overdue, and a great Mario game.
3. Scribblenauts - This game is very creative and really shows off the potential of the DS touchscreen. Very addictive gameplay, since I'm always trying to come up with crazy new objects to interact with in the game.
4. Yoshi's Island DS - Like 2-D Mario games, the nostalgia facotr is huge for me on this one. Yoshi's Island was one of my favorite games on SNES, and even though this one doesn't quite live up to that game, it's still a great game on its own merit and allows for many hours of gameplay, especially if you want to go for all the collectibles. Though a little unnecessarily difficult at times, but still a solid Yoshi's Island experience overall.
5. Castlevania: Order of Ecclesia - Was never the biggest Castlevania fan, but this is the game that has made me want to go back and play the old games that I missed out on. I like the RPG elements mixed in with the fun addictive side scrolling hack and slash gameplay.