In Julius Caesar, who is the protagonist?

Most audiences and readers tend to regard Brutus as the chief protagonist. He is the one who most elicits our sympathy, and he fits Aristotle's definition of the tragic hero, perhaps more fully than other, much more famous characters in Shakespeare. At the close of the play, even his enemy Antony pays tribute to him in a manner that does not have a true parallel in any of the tragedies generally considered Shakespeare's greatest (i.e., Hamlet, Macbeth, King Lear, and Othello).
Unlike those plays, Julius Caesar does not have a single character who is the absolute focus of the action and themes. Why, one may ask, is the play not named Brutus? As a political drama, Julius Caesar shows us a thematic complexity different from that of the other major tragedies. Which side is right or wrong? Is Caesar, as Antony describes him, the "noblest man that ever lived in the tide of times," or is he the ruthless tyrant the conspirators see him? Is Antony good, evil, or both? In his cry to "let slip the dogs of war," do we see a man selfishly wishing to cause havoc for private revenge or one who wants war for a legitimate purpose—for the good of Rome?
These questions cannot be answered without generating a huge amount of controversy. One can say that this is true, perhaps, about anything in literature, but in this play Shakespeare deals with timeless questions that affect our view of multiple characters and their function in one of the most crucial episodes of history. Even Cassius is an ambiguous figure—not the villain he might appear on the surface. For most people, Brutus does stand out as the most admirable man in the cast of history-makers, but Antony and Caesar also can lay claim to the role of chief protagonist, depending on the weight we give to those intangible factors that endow the action of the play with its deeper meanings and its timelessness.