Which actor played Dr. Who for the longest time?

The Fourth Doctor is an incarnation of the Doctor, the protagonist of the BBC science fiction television series Doctor Who. He was portrayed by Tom Baker for seven consecutive seasons, which remains the longest tenure of any actor to portray the lead, counting both the classic and modern series. He was considered to be the most recognisable and iconic incarnation of the Doctor both in the United Kingdom and internationally.

Within the series' narrative, the Doctor is a centuries-old Time Lord alien from the planet Gallifrey who travels in time and space in his TARDIS, frequently with companions. When the Doctor is critically injured, he can regenerate his body, changing his physical appearance and personality in the process. Baker portrays the fourth such incarnation, a whimsical and sometimes brooding individual whose enormous personal warmth is at times tempered by his capacity for righteous anger. His initial companion was intrepid journalist Sarah Jane Smith (Elisabeth Sladen), who had travelled alongside him in his previous incarnation, and she is later joined by surgeon Harry Sullivan (Ian Marter). His later companions included robotic dog K-9, savage alien warrior Leela (Louise Jameson), Time Lady Romana (Mary Tamm and Lalla Ward), alien teenage aristocrat Nyssa (Sarah Sutton), teen genius Adric (Matthew Waterhouse), and Australian flight attendant Tegan (Janet Fielding).