It depends on what you want.
Super Smash Bros Brawl and Mario Kart Wii are both incredible Multiplayer games. But if you're looking more for Online play, get MK, because Brawl's Online play is... horrible, at best.
If you're more of a single player guy, and you just enjoy playing alone, without friends, get Super Mario Galaxy.
But the Wii in general is a great multiplayer console. If you have a lot of friends that come over, and you have either a lot of Wii remotes or Gamecube controllers, you can't beat Smash Bros.