Frazer Hines
Hines' "Doctor Who" debut came in 1966, when he was cast to play the part of Jamie McCrimmon, the companion of the Second Doctor (played by Patrick Troughton). Originally intended as a one-off guest character, Jamie joined the regular cast and appeared in the series from 1966 to 1969. Hines reprised the role in a cameo in the 20th anniversary serial "The Five Doctors" (1983) and as a guest star in "The Two Doctors" (1985). Hines would appear in 117 episodes of "Doctor Who" in all—more than any other "companion" actor in the history of the series. The only actors appearing in more episodes are those who played the first four Doctors. Many of the episodes featuring Jamie no longer exist in the BBC's collection.
David Bradley (English actor)
In 2017, in the final episode of the tenth series of "Doctor Who", "The Doctor Falls", Bradley returned to portray the First Doctor, having previously been portrayed by Hartnell, who originally played the character. He reprised this role in the 2017 Christmas special, "Twice Upon a Time". This makes him the third actor to play the role in the television programme, after William Hartnell and Richard Hurndall since the premiere of "Doctor Who" in 1963, and at the age of 75, he is the also the oldest actor to play the role of the Doctor.