Who is the longest-serving Doctor in the history of the show?

Tom Baker is the longest-serving actor to portray the Doctor in the history of the television series Doctor Who. He played the role for seven consecutive years, from 1974 to 1981, and appeared in a total of 172 episodes. Baker's portrayal of the Fourth Doctor is widely regarded as one of the most iconic and memorable in the show's history.

Baker's tenure on the show was marked by a number of significant events and storylines, including the introduction of the character of Sarah Jane Smith and the departure of the Doctor's longtime companion, Leela. He also faced off against some of the show's most memorable villains, including the Daleks, the Cybermen, and the Master.

Since leaving the show, Baker has remained a beloved figure among Doctor Who fans and has made numerous appearances at fan conventions and other events. He has also continued to act in a variety of film and television projects, and has even reprised his role as the Fourth Doctor in a number of audio dramas and other spin-off media.