All of these games are really fun and of course don't last long enough in my mind; but Wii Music wins hands down in my mind because of all the creative potential that it boasts. You can make all sorts of music for it (although sometimes it would be nice if singers and rappers actually said words ^ ^).
And the NES horn is just simply awesome.
It actually keeps me coming back unlike WiiFit, which you're so supposed to do every day.
What's yours?