Double Dash. I found that it had better/fun items and Blue shells only appear in 150cc/mirror unlike Mario Kart DS and Wii. Plus it had the best battle mode imo and you can do grand prix as long as there's 1 or 2 screens :P
Wii, it is very fun to play and has very fun online multiplayer. DS, I also like but I don't like how when someone is in second and you push them off and go into last, they just catch up and you see they are back in second place.