I'm still scratching the surface of DS games having only gotten my DS this week, and I've been testing lots of games, but I can safely say I've played New Super Mario more than any other. It's unbelievably good. For someone who's been playing 2d Mario games since the very start, this is such a treat. I can only hope they continue this new series.
I've also been into Picross. Although the bigger puzzles are not as fun as the smaller ones, if only because you have to zoom in and crap.
I'm training for the MK DS Tournament of GBATemp.net, and I'm addicted again
When is this tournament and how does one get in?