Although there are many heroic deeds in Revenge of the Sith, the movie's main hero goes beyond simple heroics to become the audience's chief protagonist and champion. However in this movie by itself, some of the movie's main characters do not succeed, another becomes the antagonist, while one main character shows his true face - that of a dictator who will create war to ensure his unique brand of peace.
That asks the question, what determines a hero in a film, is it merely the obvious "good guy" who fights for justice? What about someone who perseveres when the Republic is being destroyed around him? Or could the definition be stretched to include someone like the Emperor who doesn't care if he's a despot and simply is trying to create - by his own rules - order for the galaxy? You'll have to decide that for yourself when you vote.