Mario Kart Wii! Well, it's great for online play, you can join in a game with multiple people you have on your friend list alongside random players from all over the world.
As for local multiplayer I can only vouch for 2 player GP racing, I've never really cared for 4 player battles in any Mario Kart ever, really. Still, if you've got other real life friends with Wiis you can get a big game going online, 2 people per console can hop online.