I also don't think I could pick just one, but here are a few:
All Around the World (or Around the Universe, or Traveling Matt, however it's known to people)
Bring Back the Wonder (yeah it's a little sad, but I like it)
Magic Be with You (a very fitting end to the show)
There are others but I can't think of them right now. Music was such a central part of that show, it was all so great!