Snowball's intelligence, hard work, and competency make him an excellent leader. We learn early on that he is highly intelligent, for he is the best writer among the pigs. Orwell also shows, subtly, that Snowball is willing to work hard in a way Napoleon is not. For example, it is Snowball who climbs the ladder and paints the Commandments on the side of the barn. Snowball also creates Animal Farm's flag and energetically organizes the many animal committees.
Snowball takes the time to educate himself on what he needs to know to be a good military leader. For example, he finds and reads an old book about Julius Caesar's military campaigns. This reading helps him organize an effective defense when Farmer Jones and the humans attack Animal Farm. Without his clever strategy, Animal Farm might have been defeated almost as soon as it began.
While not entirely without some notion of privilege, he is also a true believer in the ideals of the revolution: he is firm with Mollie about no sugar and no ribbons. Animals have to be animals, he insists, not behave like humans.
Snowball studies a farm journal and plans innovations to improve crop yields on the farm. He also makes plans to build a windmill, with the idea of bringing material benefits to the animals.
Napoleon, in contrast, is merely a crass political opportunist who does no work for the common good and has no new ideas. All he is interested in is amassing power for himself and a very few of his cronies. It is easy to imagine that had Snowball's intelligence, ability to plan, and dedication won the day, Animal Farm would have become a much better place.