According to critic Northrop Frye:
Tragic heroes are so much the highest points in their human landscape that they seem the inevitable conductors of the power about them, great trees more likely to be struck by lightning than a clump of grass. Conductors may of course be instruments as well as victims of the divine lightning.
I've always taught Creon to be the main tragic hero. He is the "highest point in the human landscape" because he is the king. His tragic flaw appears first: he decides not to bury Polyneices. Like Oedipus, Creon has the angry confrontation with Teireisias, the voice of reason in the play and the spokesman for the gods. Like Oedipus, Creon is left to suffer at the end. Like Oedipus, Creon suffers most from hubris, the arrogance of power. He loses the most: his son, wife, niece, and respect of his people.
So, if Creon is the "great tree" that gets "struck by lightning," then Antigone, Heamon, and Eurydice are the "clump(s) of grass" and the "victim(s)" who suffer as a result. Antigone is important: her death haunts the play the same way that Caesar's death haunts Brutus, but Creon is the main lightning rod of suffering in the play.
I think that you can find analysis supporting one or both characters as operating as protagonists. For my money, I have to go with Antigone. I think that because the title of the play bears her name would be one reason. Also, I think that she is the moral center of the play. It is through her character that we, as the reader, begin to see the drama unfold between the law, as represented by Creon, and justice, as seen in her. It is through her insistence that this debate is even waged. Creon would be happy not having this argument, while Antigone is the one who forces the issue. At the same time, I would say that she is the protagonist for while she is central to the work, her importance is because of her insistence in the call for social change and demand for the articulation of things should be as opposed to what they are. This ability to transform reality is what makes her the protagonist in my mind.