5. Romana II (1979-1981)
At the beginning of season 16, the Doctor was tasked with training a young Time Lady named Romana, played by Mary Tamm. At first she was portrayed as being brash and haughty, thinking herself better than the Doctor. But by the end of the season, actress Mary Tamm decided to take her leave. The crew realized since she was a Time Lady, they could simply have Romana regenerate. Enter Lalla Ward, who played Romana II. After a rather
controversial regeneration scene, she quickly settled into the role and gave us a much more pleasant, charming Romana than we'd seen before. Not only that, she was such a hit with audiences she wound up holding her role for a total of 46 episodes.