In Macbeth, there is no question in my mind that Macbeth is the villain. To begin with, Macbeth kills Duncan. In this, he not only murders the King (which is a sin against heaven), but in killing Duncan, he has killed his cousin, and a guest in his home (which was considered especially uncivilized: people were guaranteed safety as guests even in their worst enemies' homes). Besides Macbeth also murdered his friend Banquo, tried to murder Banquo's son, and kills all of Macduff's family, including wife and children. He "traffics" with witches, as well.
While Banquo is a brave fighter and an honorable man, he is killed before he has the opportunity to be seen as a hero in face of Macbeth's actions. However, Macduff shows himself to be extremely heroic. Not only was he a loyal and fierce supporter of his king, he also is a man who is not afraid to take a stand. When Macbeth invites (orders?) Macduff to attend his banquet, Macduff has his suspicions about Macbeth and his part in Duncan's death. He refuses to attend. He visits Malcolm in England to join him in his attempt to regain the Scottish throne. And he pays the ultimate price of losing all of his family members when Macbeth decides to punish Macduff. It is at this point that Macduff moves with Malcolm's army against Macbeth, and Macduff confronts Macbeth, to fight to the death.
Rats...amy-lepore stole my first reaction to this question. Of course it is. Banquo is heroic in several senses: he's loyal to his king and to his friend, he's truly glad for the apparent blessings bestowed on Macbeth, he thinks before he speaks (as when he suspects Macbeth has killed Duncan but wants to give Macbeth a chance to speak), he's brave in battle...and more. Macduff is also heroic in that he places the good of his country even above his own welfare. They're both worthy of the title of "hero."
In your question concerning Shakespeare's Macbeth, you don't ask about Banquo or Macduff being "tragic" heroes. I'm assuming that's not what you are interested in, since no possibility exists that either of those thanes are "tragic" heroes. Macbeth is the tragic hero/protagonist of the play. An argument can't be made for anyone else.
You do ask whether or not Banquo or Macduff might be a hero. Banquo is honorable and serves as a foil to Macbeth. He is in a similar situation to Macbeth (regarding the witches' prophecies), but handles the situation as it should be handled. But he is also a bit naive and too trusting of Macbeth. Macbeth orders him killed, and the orders are carried out. He loses. Banquo is not the hero.
Macduff, in contrast, suspects Macbeth of treachery the minute Macbeth kills the grooms. As Macbeth's tyranny grows Macduff travels to England to unite with Malcolm against Macduff, thereby avoiding Banquo's fate. He faces off against Macbeth and defeats him, doing to Macbeth at the end of the play what Macbeth does to other traitors at the beginning.
Macduff is the hero of the play, though he is not the tragic protagonist.
Of course it is. From the very beginning, the witches told Banquo that he was "not so great, but greater" than Macbeth and "not so happy, but happier." Banquo "fears" that Macbeth's success was "gotten most foully" and suspects his dear, once-honorable friend of foul deeds to make King. Macduff also suspects Macbeth's quick ascension to King, and does not attend the coronation. Meanwhile, back at the castle, Banquo's ghost shows up to put Macbeth in his place. Macduff also chooses not to attend this banquet celebration as another snub to someone he does not accept as worthy of the title of King of Scotland.
Fleance has fled to England where most of the "good" people who are rebelling against Macbeth have gone. Fleance and his children will bring on the other prophecy by the witches--Banquo will not be King but many Kings will come from his children and his children's children.
In the end, it is the "good" army from England which defeats the evil army supporting Macbeth. Macduff is the honorable and undefeatable warrior in this battle, and the killing of Macbeth falls to him as he avenges his wife, children, and entire household.
In order to answer this question with proper critical analysis, you must rely on more than opinion to create your argument. You must define the term "hero" as it applies to Macbeth specifically and to a classical tragedy in general. Here is the Enotes definition of a tragic hero:
A tragic hero is the main character in a tragedy. The modern use of the term usually involves the notion that such an hero makes an error in his actions that leads to his or her downfall.
This downfall is caused by the tragic hero's flaw, referred to as a tragic flaw. So, hold Banquo and Macduff up to this test. Is either of them the main character of the play (which means that the majority of events in the tragedy would revolve around his actions)? Does either of them have a tragic flaw that leads to his downfall?
Shakespeare also called this play Macbeth for a reason. Nearly all of his great tragedies (excepting the controversially named Julius Caesar) for the protagonists of the plays: Hamlet, King Lear, Othello, Romeo and Juliet and Macbeth. It would take a very strong and compelling argument to place either Banquo or Macduff as the tragic hero of the tragedy called Macbeth by Shakespeare.
And so, if we can agree that Macbeth is the hero of the play, than that would make him the play' s protagonist. It would be impossible for him to be the play's protagonist and villain (antagonist) at the same time.
For more on the tragic hero and a superlative essay on Macbeth as tragic hero, please use the links below.
THE PLAY MACBETH WAS PROBABLY A TRIBUTE TO WHICH KING?