Mao Zedong

Mao Tse-Tung was the communist leader of China from 1949 to his death in 1976. In 1936, the Third Doctor joined Mao Tse-Tung on the Long March — during which Mao allowed the Doctor to call him Tse-Tung. (DW: The Mind of Evil)

In an alternate universe, a version of the Third Doctor said he'd only known Mao as a librarian, but it was indicated he'd known at the time of the atrocities Mao would commit in the future. (BFDWU: Sympathy for the Devil)