Personally, I am inclined to think that the hero is one who finds himself at the turning point of destiny. The hero is poised upon a balance, and with a little nudge, he can tip the scales. Hence, George Washington could be considered a hero, but Tom Sawyer could not. There may be many people who make the hero's journey possible, but the final choice rests with the hero. Mentors, side kicks, and prophets all have their own unique roles to play, but in the end, it is Frodo who must bear the ring.
Furthermore, the hero must choose virtue. If he does not, he is a villain. In this sense, both the hero and the villain are the chosen ones. Sometimes they serve as the representatives of the forces at work in the story. They are the anointed, one by Heaven, the other by Hell. Aeneas is an example of the divine hero, Voldemort is an example of the infernal villain. It is not that Voldemort actually makes a deal with the devil, but that he embodies the ideas that are the source of the books evil. And there is an idea that demons and angels are ideas, perhaps there is not so much difference between being possessed by a bad idea and being possessed by a demon.
Anakin Skywalker is a good example of all these concepts. He is the chosen one who becomes a villain and brings down the galaxy, then later he uses his unique position to restore it. The point of the hero's journey is to become able to choose well when the moment upon which all things hang has arrived. Joan of Arc, Harry Potter, and Prospero all have to decode the fate of their world. As the knight in Indiana Jones said "Choose wisely."