Most sources point to either/both Napoleon or Snowball as the main characters of Animal Farm, since they are instrumental in the story.
Napoleon was a large, rather fierce-looking Berkshire boar... with a reputation for getting his own way. Snowball was... quicker in speech and more inventive...
At the beginning, they are equal in their leadership, since the basic purpose for the farm is to have all animals equal. However, while Snowball remains committed to the original philosophy, Napoleon quickly figures out that all work will be divided, and so he will be expected to pull his weight alongside the others. Instead of acting directly, Napoleon works to prop himself up as an infallible leader, smarter than all the other animals. Snowball does not change; his idealism becomes his undoing, and he is driven off the farm by Napoleon's dogs (who have been trained since birth for violence and unerring loyalty), and is later used as an all-purpose scapegoat. Snowball's ultimate fate is never clarified.
Napoleon, on the other hand, understands that Old Major's philosophy cannot work without every animal being fully committed, and since he and the other pigs wish to gain the benefits without the work, he sets himself up as a dictator. By controlling the population with propaganda and fear, Napoleon can prosper while the other animals work -- they all believe that their revolution has borne good fruit instead of becoming a dictatorship. Napoleon's shift from Comrade to Ruler is clearly seen in his speech at the end of the book:
They had been credited with attempting to stir up rebellion among the animals on neighbouring farms. Nothing could be further from the truth! Their sole wish, now and in the past, was to live at peace and in normal business relations with their neighbours.
(Orwell, Animal Farm, msxnet.org)
Since Old Major's original idea was to create revolution around the country, Napoleon's words directly contradict everything the other animals have come to believe. Instead, Napoleon sets himself up as a surrogate human, and since he now knows that the animals are capable of implementing revolution, he is a harsher and more brutal master than Farmer Jones had ever been.