I kind of like the puzzle games for the DS. My favorite is Tetris. It's an awesome new installation of the game that is way addicting. Good luck finding a copy though. The Brain Age games are fun at first but then they suck. At least for me. I only have Brain Age 2 and the only reason I keep playing it is for the Virus Buster/Dr. Mario mini game in it. It's really fun. That and Sudoku. It's only $20 bucks so its worth it. I hear Picross is a lot of fun as well. I plan to get that. Also the DS version of Bust A Move.
A new game that just came out for the DS and got a lot of hype was a game called, The World Ends With You. I plan to get that as well. Of course MarioKart is a lot of fun as are the CastleVania games. I only played the first, I still want to get Portrait of Ruin. Mario and Luigi: Partners in Time, Zelda Phantom Hourglass Worms: Open Warfare, Advance Wars. Remember the DS has Wi-Fi capability which makes it a lot of fun.
Elite Beat agents is also a blast to play.