Best friend

Bill Pollock identified dogs as "man's best friend." (TV: A Girl's Best Friend)

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)

Sarah Jane Smith 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)

The Fourth Doctor identified K9 Mark I, a robotic dog, as his second best friend, which K9 affirmed. (TV: Underworld, The Invasion of Time) He later called K9 Mark II his best friend. (TV: The Pirate Planet)

K9 Mark I's own creator, Professor Frederick Marius, regarded him as his best friend. (TV: The Invisible Enemy)

For his help in measuring a wormhole, the Tenth Doctor called Malcolm Taylor his new best friend. (TV: Planet of the Dead) Just prior to the final reward preceeding his regeneration, the Doctor referred to Donna Noble as his best friend. (TV: The End of Time)

Clara Oswald claimed to the Cybermen that she and the Twelfth Doctor as 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)

Jack Harkness recalled that, when he was a boy, he went to war alongside his best friend, who 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)

Behind the scenes

In NOTVALID: The Curse of Fatal Death, the newly regenerated Thirteenth Doctor identifies Emma, who was previously engaged to her ninth incarnation, as her best friend.