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 Fourth Doctor identified K9 Mark II as his second best friend. (TV: The Invasion of Time)

Just prior to the final reward preceeding his regeneration, the Tenth Doctor referred to Donna Noble as his best friend. (TV: The End of Time)

Amy Pond called the Eleventh Doctor her best friend. (TV: Day of the Moon)

Following her death, the Twelfth Doctor referred to Clara Oswald as his best friend. (TV: Heaven Sent)

The Thirteenth Doctor referred to Ryan Sinclair, Yasmin Khan and Graham O'Brien as her best friends. (TV: Resolution)