Duncan, King of Scotland. Macbeth, after he hears from the witches that he is to be king, plots with his "dearest partner of greatness", Lady Macbeth, his wife, and murders the king when he is staying at his castle at Glamis.
You might also be clever-clever and answer the question by saying that, at the very end of the play, Macduff kills Macbeth to get him off the throne, and remove his power. Malcolm, Duncan's son, then takes over the throne.
Hope it helps!