MISSION: IMPOSSIBLE may have opened with more different main title sequences than any other dramatic series in television history. Because the show's editors teased what was to come in the next hour by cutting clips into the opening, MISSION: IMPOSSIBLE technically aired 171 different main titles--one for each episode.
MISSION: IMPOSSIBLE is most likely the most cleverly written drama in American television history. Each episode was a tightly constructed caper or con featuring government agents with the Impossible Missions Force attempting to overthrow, capture, trick, rob, fool, or even force to be killed a series of foreign spies and dictators (in later episodes, the IMF concentrated on the Mafia and other American criminals). Far wittier and more complex than the Tom Cruise movies that tarnish the MISSION: IMPOSSIBLE name, the 1966-73 television series created by Bruce Geller could never be accomplished in today's TV environment, because it requires too much intelligence on behalf of both its makers and the audience to create.
Even if you've never seen the show, you probably know its theme, one of the best pieces of music ever created for television. Composed by jazz musician Lalo Schifrin, who also scored DIRTY HARRY and ENTER THE DRAGON and wrote the MANNIX theme, in 5/4 time, the theme was often woven into the episodic scores.
Here's a title sequence from the fifth-season episode "Flight." You'll note original stars Steven Hill (who was forced out after the first season) and marrieds Martin Landau and Barbara Bain (who left after the third) have been replaced by Leonard Nimoy (who was just coming off STAR TREK), Lesley Ann Warren, and, of all people, Sam Elliott (!), who was basically alternating episodes with original co-star Peter Lupus at this point (fans loved Loop and hated Elliott, and Peter eventually won back his job fulltime).