I love to work on the antagonist. Every writer knows that his or her hero can only be as great as his or her nemesis. I'd say to create the perfect antagonist you got to know what your protagonist wants to achieve and what he has to learn in your story. The antagonist must have the same goal as the hero, so there can be only one winner in their fight. Also, I think it is essential that all the odds are on the antagonist's side. David challenges Goliath. Even more important is the learning aspect. The right antagonist exposes the hero's weakness, this way forcing him to admit to himself that there's something wrong with him. He will overcome the antagonist only after such self-relevation. Finally, the antagonist should represent a value system that is antagonistic to the protagonist's. Let's say, our hero fights for individual freedom. Then his or her antagonist should defend the interests of the general public, the state, law and order. I have to understand his or her motives to give the antagonist an authentic voice.