Inifnite Spin broke Tetris DS for me.
Snaking broke Mario Kart DS for me.
I liked Lost Magic. Reminded me of Canon Fodder, plus it has Studio Ghibli character designs. You actually have to LEARN magic or you will not like this game. Excellent use of the touch screen (i.e. This is not possible on any other system).
This game is better than any review I read gives it credit for. Because there is a time limit on the stages you will be thinking "out of the box", alot. If you run out of time on a stage it's because you're doing it wrong, nothing else. Still though, the game is pretty tough. Right up there with Trauma Center, which is why I think I loved this title so much.
The game does ease you into it, the first couple hours are nice and brisk, then it ramps up from there. You can also do Wi-Fi play with this, though I don't know anybody else with this title. So I've not done it.