In terms of history of Greek mythology, it would be Helen of Troy. She was described by Christopher Marlowe as having "the face that launched a thousand ships."
In terms of actual people, probably Cleopatra.However, that is subject to debate. Here's what it says in Wikipedia re: how Cleopatra was considered in terms of looks...
In most depictions, Cleopatra is put forward as a great beauty and her successive conquests of the world's most powerful men is taken to be proof of her aesthetic and sexual appeal. In his Pensées, philosopher Blaise Pascal contends that Cleopatra's classically beautiful profile changed world history: "Cleopatra's nose, had it been shorter, the whole face of the world would have been changed."
Plutarch, however, describes her in one passage of his biography Life of Antony thus: "For her beauty, as we are told, was in itself not altogether incomparable, nor such as to strike those who saw her". According to Plutarch, what ultimately made Cleopatra attractive was her wit, charm and "sweetness in the tones of her voice". And yet Plutarch remarks in an earlier passage that "judging by the proofs which she had had before this of the effect of her beauty upon Caius Caesar and Gnaeus the son of Pompey, she had hopes that she would more easily bring Antony to her feet. For Caesar and Pompey had known her when she was still a girl and inexperienced in affairs, but she was going to visit Antony at the very time when women have the most brilliant beauty".
Cassius Dio, a noted Roman historian, agrees: "For she was a woman of surpassing beauty, and at that time, when she was in the prime of her youth, she was most striking; she also possessed a most charming voice and a knowledge of how to make herself agreeable to every one. Being brilliant to look upon and to listen to, with the power to subjugate every one, even a love-sated man already past his prime, she thought that it would be in keeping with her rôle to meet Caesar, and she reposed in her beauty all her claims to the throne".
