Lists by GusF
Richard Dean Anderson and Michael Shanks are the only actors to appear in the pilots of all three series.
Five of them were made during the 1960s and four of them feature time travel.
Steve McQueen, James Coburn, Charles Bronson, Whit Bissell and George Voskovec each appear in two of the films. The first three all appeared in "The Magnificent Seven" and "The Great Escape", Bissell appeared in both "The Magnificent Seven" and "The Time Machine" and Voskovec appeared in both "12 Angry Men" and "Somewhere in Time".
Tom Baker (possibly!), Peter Davison, Paul McGann, the late Elisabeth Sladen and John Leeson are the only actors to play the same character in both series.
Some would have been great, some would have been bad and some would have been so bad, they would have been good.
Of these 72 series, only 16 have 100 or more episodes and only two of those ("Stargate SG-1" and "Law & Order") have 200 or more episodes, which shows that I do have something resembling a life!