I would like to know who the protagonist in the story is. Is it Antigone or Creon?Both characters play a leading role,but who is the protagonist?...

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.