What moment might be called the climax in Act 5 of Shakespeare's Macbeth? What is clear to the audience and to Macbeth at this point?

The most climactic moment of Act 5 is when Macbeth and Macduff face off, and Macbeth realizes that he is doomed. 

Macbeth really takes an emotional roller coaster during this play.  He goes from really wanting to be king to killing Duncan and then worrying that he will be found out.  After that, he descends into a madness of guilt and paranoia.  By Act 5, his wife has died and soldiers are pouring at him from all angles.  

The witches told him that he needed to worry about the forest coming to him, and it turns out that, in a way, it does.  Malcolm has his men cut down branches to hide their approach.  He is also told that no man born of woman can hurt him in one breath and to beware Macduff in another.  Thus by the time he meets Macduff in battle, he is a mess. 

The climactic or most exciting and meaningful moment of the play is when Macduff confronts Macbeth, and clears up the mystery.

MACDUFF

Despair thy charm;
And let the angel whom thou still hast served
Tell thee, Macduff was from his mother's womb
Untimely ripp'd.

MACBETH

Accursed be that tongue that tells me so,
For it hath cow'd my better part of man! (Act 5, Scene 8) 

It turns out that Macduff was born by some kind of Caesarean-section, so he technically was not born in the traditional sense.  There you go.  It is clear to the audience that Macbeth is completely deflated after hearing this. He feels betrayed and loses his fighting spirit.  It is not hard for Macduff to take him out.  He does so by beheading him. 

There is not much more to the play after that.  Malcolm’s soldiers make short work of whatever remains of Macbeth’s forces, and Malcolm declares the tyrant killed and the war over, taking his rightful place at the throne.