Old Major was one of the rulers because he came out with the idea of animalism, when he was dying. All the animals in the farm were ready to follow his advice and this is what they did, although old major was not the one who led them.
Boxer also might have made a good ruler because he was intelligent and hard working. Everything he does, he does it for the good of the farm and he is not selfish. He is respected by all the farm animals and when they take him away to a slaughter house after he is injured during work they all felt deeply sorry for him.
Snowball could also have been a good leader because he was creative and not as self-centred as Napoleon. He came with the idea of the windmill, which Napoleon rejected at first and later said that it was his idea. When he was chased out of the farm, he is blamed for all the things that happen in the farm. Half of the farm was with him while the other half was with Napoleon.
Napoleon later became the leader of the farm. He was very selfish and everything he does, he does it for his own comfort. At first, he respects the principles of animalism, but later he manipulates them in his own favour. In the end of the story Napoleon becomes worst than Mr. Jones.