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.
Animal Farm cannot be said to follow the pattern of most of the novels of its time. Since it fictionalizes the rise of Stalinism, the search for the story's "true climax" may misguide the reading. There are undoubtedly at least three startling turning points that could be viewed as climaxes. The first is Snowball's ousting by Napoleon, the second the revelation that the missing puppies have been trained to hound and kill dissenters, and the third the moment when, at the end of the book, the dispossessed animals witness the reunion between men and pigs and are unable to distinguish between them.
Each of these moments will echo differently in different readers. Much as some critics try to standardize analytical approaches and provide "the" key to interpret literary works, the last word lies with the reader. It is up to him/her to appropriate and complete the writer's work.
If you are bent on identifying what you call the "true climax", turn to your own reaction to the turning points mentioned. My feeling is that the first two pave the way for the last.