There are only two possible answers to this question. Either the most evil person would have to be Macbeth or Lady Macbeth. The three witches are minor characters and do not seem so much evil as insane.
Lady Macbeth must be considered the most evil because she knows she is evil, wants to be evil, and wants her husband to be equally evil. She is also duplicitous, an expert practitioner of villainy. Her husband does not measure up to her in wickedness, and she knows it. He can't conceal his guilt and his remorse. She would probably love him more and respect him more if he were more evil. In her soliloquy in Act 1, Scene 5, she says of Macbeth:
Yet do I fear thy nature;
It is too full o' th' milk of human kindness
To catch the nearest way. Thou wouldn't be great,
Art not without ambition, but without
The illness should attend it.
By "illness," she apparently means the pure evil character which she herself possesses. She is not only the most evil character in Macbeth, but possibly the most evil character Shakespeare ever created.