The Master

He first appears in two guises: a gentle, absent-minded scientist called Professor Yana tending to refugees at the end of the universe, and Harold Saxon, a telegenic British Prime Minister riding a wave of public adoration. Beneath both masks is the Master, a renegade Time Lord and the Doctor's oldest rival, delighted at last to be recognised.

John Simm plays him as barely contained, all beaming charm and quick wit curdling into something manic and cruel in the space of a sentence. The writing lets him be genuinely funny before it lets him be frightening, so the moment his mask drops feels less like a twist than an unleashing.