One of the most appealing aspects of Doctor Who is that the reality created within the show is conceptually strong enough to handle the replacement of every single actor in it. This means that fandom and appreciation can be comfortably spread over the entire 50-year run. Second Doctor fans can debate their preferences with Sixth Doctor fans, and no one can be definitively wrong or right.
That said, there are a few good reasons why Tom Baker, the Fourth Doctor, often comes out as the most popular of all of the incarnations in the classic series, an affection that sometimes rivals the more recent Doctors too.
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