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You could argue either one. You'd have to marshall your reasons for why, and then argue the point. What's the definition (or "a" definition, because there are several interpretations) of a tragic hero? Draw up that list. Now, given the action of the play and your interpretation of it, would you consider Antigone to better fit that definition or Creon?
I thought a lot about this and listed why, but the one thing that got me stuck with Antigone is that I don't think she ever realises her flaw which is one of the main elements for a tragic hero besides hamartia, having a downfall, initially being in a high ranking position, etc.
I don't know what level you are working at, but there are many interpretations of "hamartia." The text we are working with for the definition of the "tragic hero" is Aristotle's _Poetics_ and there is wide disagreement on the interpretation of that term. The _Poetics_ was a set of rough notes Aristotle used to lecture from, and many points in the text are not at all clear. Many scholars read "hamartia" to mean simply error. An error in judgement, an error in action. It's a plot point, in this reading, not a character trait. It is also well worth noting that the tragedians themselves were working long before Aristotle put together his theories on tragedy (which we might also think of as his theories on literature), that they didn't necessarily construct plays that followed his precepts, that he focused on only a very few of the many hundreds of plays that were written, and that he had a particular bent of mind. Aristotle's analysis is so very Aristotelian, if you get my drift. His ideal trajectory fits only a very few plays. But all of that being said, if you've been given a particular definition that you must adhere to, then you will need to go with that. If you cannot argue from any evidence in the play that Antigone recognizes her flaw, then you should argue for Creon's being the tragic hero. I am too little familiar with the play at this point to speak in other than generalities. I remember the principle action, but not the details of who said what when. Some of that also depends upon what translation you're working from. Different translations can take different approaches in the rendering of ancient Greek to modern English. But you can certainly argue for Creon's being the tragic hero. It's been done, so you're in good company.