3. The Rani
The Rani is hardly the most popular villain in Doctor Who, having appeared in the canon at around the time people noticed the drop-off in quality which eventually led to the series' hiatus. Yet she still has her fans, and its easy to see why. A Time Lady who was one of the Doctors oldest school-friends, Her history is slightly ridiculous she was banished from Galifrey for creating a giant rat which bit the Lord President, who had to regenerate but her motives remain a fertile source for any potential plot. Namely, the woman loved to research, and loved to research on Earthlings. As well all know, mad scientist stories always have their place on Doctor Who, with the new series using things like The Lazarus Experiment and the hospital ran by The Sisters of Mercy. Both of these experiments involved tearing up the scientific rulebook and doing something abominable, something which could bring a level of fury out of the Doctor which Tennant seemed able to pull off but might be just in Capaldis wheelhouse, looking at his positively ferocious and terrifying stare.