I'd have to say Mario Kart Wii because of the good graphics and controls, as well as the introduction of tricks. The game also has a huge character roster. I'm glad that Double Dash!! tied with MKWii for first though.
Really great game. I got it the day it came out (November 13) and it took me until the Friday/Saturday that followed to fully beat it. Super Mario 3D Land is a great way to start off my 3DS library, even though it does seem like a relatively short game (or I'm just a very fast gamer, or both)...