There are three pig characters in George Orwell's Animal Farm, and all three of them are strong leaders in some way. The most effective leader may not be the strongest, so you'll have to make clear your definition of "strongest."
Squealer is the pig who acts as the spokesperson for Napoleon. He's the master of propaganda, and he is able to convince the other animals of just about anything. If being a leader means being able to "turn black into white" and getting people to believe what he says, Squealer is an effective leader.
Snowball is the pig who vies for leader of the farm against Napoleon. He is more benevolent toward the animals than Napoleon, and he does seem to have the best interests of the animals in mind--again, at least compared to Napoleon. He does lead the animals to victory in the Battle of the Cowshed, but he is ultimately run off the farm. Snowball is a strong leader until Napoleon "eliminates" him as competition.
Finally, Napoleon is the pig who reigns as supreme leader of Animal/Manor Farm once Napoleon is gone. He's selfish and cruel and has nothing but his own interests to motivate him. If being in control is being a strong leader, Napoleon is it.
How one defines "leader" determines which of them is the strongest, and you'll have to decide that for yourself.
There are, in my mind, two answers to this. In many ways Snowball is the strongest leader. He is the clearest thinker, the one who is actually concerned with the welfare of the peasants, or in this case the other animals. He is careful in terms of planning things like the windmill and the animals follow him because they know he is looking out for their best interests.
But Napoleon emerges as the story moves along, not necessarily because he is out and out the strongest, but he is also ruthless and willing to go to whatever ends necessary to get things he wants. He uses propaganda and an attack force of dogs to make sure that the other animals follow him without question.