Best friend
To Clara Oswald, a best friend was someone who came to be "the closest person to [her] in this whole world". She said that she would never betray the Twelfth Doctor, whom she would always forgive and always trust, as he had for her. Clara held herself to a different standard of behaviour with the Doctor, claiming never to lie to him. (TV: Death in Heaven)
Bill Pollock identified dogs as "man's best friend." (TV: A Girl's Best Friend) K9 Mark I's creator, Professor Frederick Marius, regarded him as his best friend. (TV: The Invisible Enemy) Kelsey Hooper observed that Sarah Jane's best friend was "a metal dog with its bum stuck in a black hole." (TV: Invasion of the Bane)
Sarah Jane Smith regarded Andrea Yates, who died when they were both aged 13, as her best friend. (TV: Whatever Happened to Sarah Jane?)
Luke Smith called Clyde Langer his best friend. (TV: The Mark of the Berserker)
Ruth referred to herself and Bernice Summerfield as best friends. (AUDIO: Everybody Loves Irving)
Jack Harkness recalled that, when he was a boy, he went to war alongside his best friend, whom he had persuaded to join. Ultimately, this friend was tortured and killed by the enemy, whom Jack described as "the worst possible creatures you can imagine." (TV: Captain Jack Harkness)
Gwen Cooper referred to Jack as her best friend. (TV: Children of Earth: Day Two)
Gabriela Camara referred to Jamila Velez, after she died, as her "melhor amiga". (TV: Praxeus)
The Doctor's best friends
Sarah Jane thought of the Third Doctor as her best friend, though she feared this might sound silly. (AUDIO: The Paradise of Death) The Fourth Doctor later referred to Sarah as his own best friend. (TV: The Seeds of Doom, The Masque of Mandragora)
While travelling with Leela, the Fourth Doctor identified K9 Mark I as his second best friend, which K9 affirmed. (TV: Underworld, The Invasion of Time) After discovering that Ann Kelso was actually Anya Kingdom, and had betrayed his trust under Zaal's influence, the Doctor decided that K9 Mark II was in fact his best friend, and not just "one of [his best friends]". (AUDIO: The Perfect Prisoners) He later reaffirmed this to K9 himself, when K9 apologised for being unable to fulfil a request on low power. (TV: The Pirate Planet)
The Seventh Doctor considered Bernice Summerfield to be one of his best friends. (AUDIO: The Unknown) He and Ace were also best friends. (AUDIO: Quantum of Axos)
On introducing him, the Eighth Doctor let slip that Alistair Lethbridge-Stewart was his best friend. When Charley Pollard objected that she thought she was, the Doctor made note never again to call someone his best friend. (AUDIO: Minuet in Hell) The Doctor latter affirmed to ROSM was Charley was indeed his best friend. He was far more comfortable putting himself in danger than ignoring that she might be in trouble. (AUDIO: Embrace the Darkness) When Charley met the Doctor's former companion, Romana, he introduced Charley to Romana as "one of my best ever friends" and Romana to Charley as "one of my best friends ever". (AUDIO: Neverland)
The Eighth Doctor also called Sam Jones his best friend, (PROSE: Dominion) and Fitz Kreiner considered both the Doctor and Sam his. (PROSE: Interference) Bliss (AUDIO: Dreadshade) and Liv Chenka, two other companions of the Eighth Doctor, also referred to him as their best friend. (AUDIO: Flatpack)
The Eighth Doctor later called the TARDIS the best friend he had. (AUDIO: Divine Intervention) The Ninth Doctor also affirmed this in conversation with Alistair. (AUDIO: The Forth Generation)
In a possible future stemming from the Eighth Doctor's life, (PROSE: The Tomorrow Windows) the newly regenerated Thirteenth Doctor identified Emma, who was previously engaged to her ninth incarnation, as her best friend. (TV: The Curse of Fatal Death)
For his help in measuring a wormhole, the Tenth Doctor called Malcolm Taylor his new best friend. (TV: Planet of the Dead) Soon before his regeneration, the Doctor referred to Donna Noble as his best friend when explaining to the Saxon Master that he had left her with certain protections. (TV: The End of Time)
Amy Pond called the Eleventh Doctor her best friend, (TV: Day of the Moon, Let's Kill Hitler, HOMEVID: Good Night) a sentiment the Doctor shared. (TV: The Wedding of River Song) River Song, Amy's daughter with Rory Williams, called both her parents the Doctor's best friends. (TV: A Good Man Goes to War)
Clara Oswald claimed to the Cybermen that she and the Twelfth Doctor were mutual best friends. (TV: Death in Heaven) Later, she identified the Doctor as Missy's best friend. (TV: The Magician's Apprentice) Following her death, the Twelfth Doctor referred to Clara as his best friend. (TV: Heaven Sent)
The Thirteenth Doctor referred to Team TARDIS, consisting of Ryan Sinclair, Yasmin Khan and Graham O'Brien, as her best friends. (TV: Resolution)
