Which incident serves as the climax of the play, "Macbeth"?
Normally I think the climax is where we see the first reversal of fortune for the protagonist and/or antagonist, but I am not sure, I think that from there the plot unravels this change in narrative momentum.
You have several stories going on here--Macbeth's plans, Lady Macbeth's life and plans, Macduff's plans, the witches' plans, the state of Scotland.
For me, the biggest turning point is when Macbeth willingly decides to seek out the witches again and they give him the apparitions he demands. We know from a prior scene with Hecate that they will make these apparitions seem to be in Macbeth's favor, but that he will misinterpret them and this will be his final downfall. His character change is complete at this point and he is traveling at "full speed ahead and damn the torpedos" attitude--regardless of what happens.
It could be argued though, that the climax is when Macbeth sees the ghost of Banquo. This is the beginning of his guilt and of his physical downfall in the play.