Super Mario Galaxy
This game is not just fun, it's damned fun. Anyone who owns a Wii and hasn't experienced this wonderful franchise yet, should definitely be ashamed of themselves.
You'll find yourself controlling Mario in all sorts of new ways that you'd never even thought of. This game is a landmark, and a fine example of what ingenuity more developers should be aiming towards when it comes to designing games for the Wii. Supposedly the sequel is even better, but I'm still waiting to beat part one first. Easily one of my favorite Mario titles of all-time, and that's really saying something.