7. The Second Doctor
BBCIt can't be overstated enough - There is no rational explanation for why the show wasn't cancelled in 1966 when William Hartnell could no longer continue in the role. The decision to recast the part and carry on, from a television production perspective, is irrational to the point of insanity. If you were going to go that route - and that's a big if - you take the safe path, recast as closely as possible to the previous actor, and hope that the audience is indulgent enough to overlook it. (This is the route Bewitched chose to take under similar circumstances) Choosing instead to cast an entirely different type of actor was insanely risky. Having that actor play the character completely differently was just full on whackadoodle. Then they went for the hat trick and had the script make absolutely no effort to convince viewers that it was the same man. If you go back and read the script for Power of the Daleks (which is sadly all we can do for the moment unless the Omnirumour turns out to be true) you'll see the lengths to which the Second Doctor goes to
not confirm or deny that he's the same character. He refers to 'The Doctor' in the third person many times. He steadfastly does
not spend any time reassuring his companions that he's the same man. He just gets on with the adventure at hand and proves that he's still The Doctor through his actions instead of through words. It was courageous, insanely risky, and absolutely brilliant. And it's why we're still talking about the show fifty years later.