I just recently got WIfi at my house, so I can now hook my Wii up online. As such, I've been playing a lot of Mario Kart, and I think it's the Wii's Halo 2 as far as online games go. It's got a really nice setup and structure, and the ranking system is really intricate. I'm really liking it.
Also, is there anything better than beating Brazilians, Japanese, British, Dutch, South Koreans, New Zealanders, Canadians, Germans, or French people in some good old Mario Karting goodness?