Indeed, the previous thoughts are valid in that both can be considered tragic figures or protagonists. I think that I have already written as to why Creon can be seen as the protagonist/ tragic figure. In the interests of presenting both sides, I think there is a case to be made as to why Antigone can be seen as the tragic figure in the play. Simply put, Antigone is the agent of action in the tragedy. It is she who brings about the tragic collision between two equally desirable, but ultimately incompatible courses of action. She is the embodiment of this tragedy. On one hand, she feels honor towards her brother and her family and on the other hand, this love brings her in direct contradiction with the state. Antigone represents the fundamental tragic position of the law and justice. Creon only represents the former of these two, while Antigone brings about the foundational tragic dilemma of what happens when the law denies a sense of justice to its citizens. It is this idea that becomes the theme of the tragedy, as it plays itself out entirely throughout Antigone's belief system and how she proceeds with it. Antigone becomes the tragic figure and protagonist in the play because she is the one who seeks justice in a world that is devoid of it. I think that another reason why one can argue that she is the protagonist is because of the title. There's a reason the play is not called, "Creon." (The last reason was cheap, but you get the idea.)
It is possible to argue that this play has two tragic figures. Sophocles presents such a balanced perspective of both characters that is difficult to determine which is the true protagonist. Both suffer from excessive pride. Both are headstrong and stubborn. Both are right in their own way--Creon upholds civil law; Antigone follows the laws of gods. Each is urged by others to reconsider and compromise. In fact, Creon does change his mind, but alas, he is too late. I think it boils down to which character you sympathize with more.
Since the play is called Antigone, it could be effectively argued that she is the protagonist. Yet, Creon suffers more at the end. He is the one who realizes his mistake and tries to change. This type of realization is characteristic of a tragic hero.
Various themes are developed through this conflict: the choice between various kinds of moral codes--civil and religious, the consequences of excessive pride, the difficulty in determining when one must stand his ground or compromise.