Of course Napoleon is the leader of the farm, because of his grip on power through the use of dogs and squealer and every other method he can get his hands on, but you have a number of other leaders as well.
Boxer is in some ways a leader of the animals, he isn't considered very intelligent but he is always looked to as an example of really hard work and willingness to get the job done regardless of how he feels about it.
Snowball was another leader, a really thoughtful and intellectual one who had great plans for ways to improve life for the animals, but he was driven out by Napoleon who was threatened by his intellect.
You could also consider Old Major as a leader, though he dies and obviously loses the leadership role soon after the story opens.
The leader of Animal Farm was Napoleon,who represents the second leader of Soviet Union Joseph Stalin, Napoleon leads the farm from the beginning of the revolution. Orwell believed that although socialism is good as an ideal ,but it can never successfully adopted due to uncontrollable sins of human nature.For example , we notice that the farm had grown richer without making the animals themselves any richer except, of course for the pigs and the dogs.This is the same things happen during Stalin reign, he lived in luxury and his common peasant suffered .Another example,Napoleon slaughtered many animals because they plotted against him. Also Stalin,he was cruel dictator in Russia.