Who is the longest running Doctor Who companion?

Answer 1

Jamie McCrimmion is the longest running companion with 113 episodes. He traveled with the Second Doctor.

K-9 is in second place with 94 episodes. He traveled with the Fourth Doctor.

Sarah Jane Smith is in third place with 80 episodes. She traveled with the Third and Fourth Doctors and is one of only two companions to appear in four consecutive seasons.

Ian, Barbara, and Jo are tied in fourth place with 77 episodes. Ian and Barbara traveled with the First Doctor, while Jo traveled with the Third.

Tegan comes in fifth place with 65 episodes. She traveled with the Fourth and Fifth Doctors and, alongside Sarah Jane, is the only other companion to appear across four seasons.

Depending on how one defines longest running companion, however, Rose Tyler has appeared in 26 total serials as of 2011, as opposed to Jamie's 21.

However, the longest-serving of all companions bar none is the TARDIS. Although this fact was hinted at from the earliest days of the series, the 2011 episode The Doctor's Wife confirmed that she is sentient and, as such, could be considered a companion just as much as the humanoids and others the Doctor travelled with. Over 50 years there have only been a handful of Doctor Who stories in which the TARDIS has not appeared in some form.

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There are a hundred ways to define the parameters here and they are all valid. But to add another answer, if one goes by length of time between first and last regular appearances, then the top five are as follows:

K9 (Mark I and II together): 1,205 days

Tegan Jovanka: 1,083 days

Sarah Jane Smith: 1,044 days

Jamie McCrimmon: 918 days

Amy Pond: 911 days

Note that all of this applies only to companions seen on television. In other media, such as audio dramas, novels and comic strips, there have been companions who have equalled and, in some cases, exceeded the longevity of TV companions. In the 1997 novel The Dying Days, the Eighth Doctor refers to Prof. Bernice Summerfield as his longest-serving companion.

Answer 2

It depends which era you are talking about... During the Tennant era Rose was favorite for romance and Donna was his best friend so it depends on your point of view