Most theme songs today provide nothing more than the chance for the show to quickly introduce the cast to a first time viewer.
The theme song for The Big Bang Theory provides a 20 second lesson on the history of the universe.
"The History of Everything" by the Barenaked Ladies does not show the cast until the final frame, but still sets the tenor for the show. In fact, most of this season the theme song could be considered the highlight of the episode.
The song is tremendously catchy (more than any other current tv theme, outside of maybe the irritating "Jabba the Hutts" of Parks and Recreation) and the split second images matching the lyrics is perfect. All in all, the best current tv theme song.