Mario Kart 7 is fun, but it requires a 3DS and the game. I'm sure more users have Mario Kart Wii and the Wii console.
Another option is Mario Kart DS (I have all three games,) but I'm not sure which is more conducive to online multiplayer. I figure most people would be more likely to own Mario Kart Wii and Mario Kart DS than Mario Kart 7 though.
Also, would you like to change the title of the thread so people know what's going on inside? I know you may think that it makes people interested in the contents of the thread by making them curious. It just annoys me though, and some people will just overlook it if they don't like the meme (making the title misleading, which is even worse.)
I only looked, because I have insomnia and I moderate all threads.