I would highly recommend every game I have for the DS, so here it goes, I've bolded my "Must Gets"
Castlevania: Dawn of Sorrows - It's a great platformer, with enough variation in weapons and abilities to hold your attention for a while. The touch screen is a little tacked on, but otherwise it's a lot of fun.
New Super Mario - If you're going to choose between this and Super Mario DS, I'd go with this, it's a little fresher and the mini games are a little better.
Super Mario DS - Still well worth getting, it puts a little bit of a new spin on a game that you've probably beaten before. I really liked the multiplayer on this even if the game is a little limited.
Mario Kart DS - There is no reason not to get this game, it is my favorite entry in the Mario Kart series, and it is also my favorite multiplayer game for the DS. This game is how we spent half of our marching band bus rides this year, the other half belonging to
Tetris DS - Sure you have regular Tetris, which I can play for hours without tiring, but it also has a great multiplayer, which everyone remembers was a lot of fun at the demonstration game boy booths, but you could never do before. The download play allows you to take on up to 9 different people if I'm not mistaken, and there is much fun and cussing to be had by all. The other games are also a lot of fun. If you ever even kind of liked Tetris, get this game.
Phoenix Wright - Now, there's a thing about Phoenix Wright. Either you love it or you hate it. Fortunately, I have yet to personally meet anyone who hated it. It's silly and clever, and that's always something to be admired in a game. Get all three and beat them all, it's very rewarding.
Phantom Hourglass - It does not disappoint as a Zelda game, and everything is a lot of fun. There is one puzzle that took me three hours to do, but ti's just because I don't always think outside of the box with video games. Why do you not have this right now? Get it.
and finally
Meteos - This was my stopgap until they made Tetris DS, and let me tell you, it was a lot of fun. It's one of the faster paced puzzle games that I've played, and it's definitely worth it.