Well the obvious ones are: Mario Kart DS and NSMB Wii. Also check out Yoshi's Island DS if you liked the first one, or even if you like platformers.
Some other great games:
Castlevania - Order of Ecclesia - just has that old school vibe to it, and one of the few very challenging games I've played this gen. Good amount of content, even after you beat the game.
Rythm Heaven - Really fun and unique rythm game. Kinda like warioware mixed with guitar hero (the drum part)
Big Brain Academy - surprisingly fun, also very challenging, at least to earn the platnuims. Really works the brain..
Scribblenauts - Has a few flaws and akward controls at first, but I love the concept of solving puzzles by conjuring up virtually any item you want.
Animal Crossing - Really fits with the DS style of games
Clubhouse Games - lots of solid mini games
Meteos - If you like puzzle games check this one out, makes good use of the touch screen.
I hear the Dragon Quest remakes are good stuff as well if you like RPGs, but I haven't played them yet.
