What moment might be called the climax in Act 5 of Macbeth?

When Macduff encounters Macbeth on the battlefield, the play is drawing near its climax. Macbeth is still full of his fighting spirit, and he has just finished killing one soldier who dared to confront him. Then Macduff appears and challenges him to hand-to-hand combat. They fight. Then Macbeth pauses and says:

Thou losest labor.
As easy mayst thou the intrenchant air
With thy keen sword impress as make me bleed:
Let fall thy blade on vulnerable crests;
I bear a charmed life, which must not yield
To one of woman born.

Macduff's reply to this might be called the climax because it undermines Macbeth's morale. Macduff says:

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

This is not only a surprise to Macbeth, but it comes as a complete surprise to the audience, who realize, along with Macbeth, that he has been deceived by the three witches. It seems possible that Macduff has never mentioned the fact that he was delivered by what in those days must have been a horribly crude and painful operation which might have resulted in his mother's death. He may have kept the secret until this very moment because it was too painful for him to think about. He describes what happened at his birth as being "Untimely ripp'd." 

No doubt the actor playing Macbeth would not respond immediately to this information. There might be a pronounced silence while he digests what he has heard and realizes all its implications. Macbeth has already lose one of his assurances when Birnam Wood appeared to be moving towards Dunsinane. At last he refuses to fight any longer. He knows he is sure to lose. But Macduff taunts him and forces him to continue the fight to the finish.

Under ordinary circumstances Macbeth should be able to beat Macduff, but because Macbeth is psychologically defeated, he is defeated in combat. Macduff has the upper hand for several reasons. Macbeth is unnerved by learning that his "charmed life" is not foolproof. He was reluctant to fight with Macduff in the first place because he felt guilty for the crimes he had committed against his adversary's family. And furthermore, Macduff is so strongly motivated to kill Macbeth for revenge that he seems possessed of superhuman strength. 

In the end, Macduff kills Macbeth and displays his severed head. But the climax seems to have been reached earlier, when he says:

Macduff was from his mother's womb
Untimely ripp'd.