What single most important point was Orwell trying to make in Animal Farm?

In a literal sense, George Orwell satirizes the Bolshevik Revolution and Soviet communism. Orwell does not specifically target the communist system, but instead presents an allegory of the corruption of government in general.

Using Animal Farm, Orwell mocks the rebels of the Bolshevik Revolution. The revolutionists want to overthrow the Russian Czar because of cruel and unfair treatment. Once the Czar is defeated, however, the leaders of the revolution (now proclaimed as leaders of the Soviet Union) become corrupted with power and end up resembling exactly what they were revolting against in the first place.