Online Mariokart. Enough said
However, when you take an already amazing game and add multiplayer support of not only 7 of your friends, but for people all over the world; it only adds to the intensity. The graphics, the gameplay and fun is everything you would expect out of a Mariokart.
Even if you don't own a DS, purchasing one simply for this game would not be a stupid move. This game can keep you occupied for many hours on end, and the last race is just as exciting as the first. I give Nintendo props for keeping Mariokart as fun as they have always been.
Go buy this game, get online with it and come get beat by me :)