Please provide evidence that Macbeth is the protagonist.

To be a protagonist, typically a character need only be the main character of the text in which he or she figures.  One good clue that Macbeth is the play's protagonist is that it is named after him; he is the titular character.  Further, it is his actions, and the things that happen to him, that the text follows.  It begins with the Weird Sisters in Act 1, scene I, but even they reference Macbeth, the character, as they make plans to meet him.  From then, the play follows Macbeth as he learns the Sisters' prophecies, goes home to confer with his wife, kills Duncan, arranges for Banquo's murder, and so on.  Although some scenes do not include Macbeth -- one shows just the Weird Sisters, another Macduff and Malcolm, another Macduff's family, etc. -- even characters in these scenes discuss Macbeth or witness the result of his actions.