If Antigone is the protagonist that means Creon has got to be the antagonist. He's the force that Antigone goes up against. As we discuss above, there are some that see him as a possible protagonist. There are good reasons for this point of view, the best of these being that Creon is the one who has moments of peripeteia and anagnorisis.
We'd just like to point out, however, that it is Antigone not Creon who disturbs the world by breaking her uncle's law. Antigone drives the action, while Creon only reacts. He is constantly trying to rebuff Antigone's forward motion, rather than creating any of his own. This in our mind makes him seem like the most likely antagonist. Once again, there's room for debate. Antigone just doesn't fit snuggly into all of Aristotle's ideas.