What is Animal Farm's true climax?I believe it is arguable. I'm wondering what everything else thinks.

I would actually like to add another potential climax to
the list provided in the answer above. Let us not forget the destruction of the windmill
and the huge blow to the animals that this is. However, I would actually want to argue
that whilst the novel features many smaller climaxes, the main climax has to be the
ending, which is when there appears to be no difference between the pigs and the men.
This is the true climax that the novel has been leading towards, and it is clear that
this confusion marks the final development or progression of the pigs and Napoleon into
the tyrannical leaders that they have become. Before this point, they are despots, yes,
but their true despotism is signalled when they become indistinguishable from the very
men that they have supposedly been opposing through the entire
novel:

Twelve
voices were shouting in anger, and they were all alike. No question, now, what had
happened to the faces of the pigs. The creatures outside looked from pig to man, and
from man to pig, and from pig to man again: but already it was impossible to say which
was which.

The sense of
terrible completion with which this ending terminates the story shows that this has to
be the central climax, as both we and the animals realise the true nature of the pigs
and what they have been trying to do through their "rebellion" and how they have
carefully shaped it.