There are two main reasons for portraying Snowball as "good" compared to the other pigs. First, as Napoleon furthers his own plans to take over the farm, it is helpful from a story perspective to have opposing forces; for a time, Snowball's reasonable measures hold Napoleon's greed in check, and the reader ends up sympathizing with Snowball (who is not seen again) when he is expelled. Snowball is also willing to defend the farm against intruders, while Napoleon is nowhere to be seen during the Battle of the Cowshed.
The other reason is that Snowball represent the "good" side of Communism, insofar as it has a good side. His ideas are for true equality, living in harmony with others and without exploitation of any sort.
Snowball had made a close study of some back numbers of the Farmer and Stockbreeder which he had found in the farmhouse, and was full of plans for innovations and improvements. He talked learnedly about field drains, silage, and basic slag...
(Orwell, Animal Farm, msxnet.org)
Because he is actually trying to attain the utopian ideal as presented by Old Major, Snowball represents the possibility of success without a ruling class. If his plans had gone through, and if Napoleon had been killed or simply not had as much sway, it is possible that the farm would have prospered as Old Major intended, instead of falling to a dictatorship. Snowball is the voice of reason and so is "better" in a purely objective sense.
I think that Orwell has some distinct motives in showing Snowball as brave. The first would be to validate his own rejection of Stalin, in the form of Napoleon. Orwell was a devoted follower to the principles and theory of the Russian Revolution. The idea of democratic socialism resulting in a transformation of reality is more in line with Trotsky's thought than anything that Stalin put forth. In depicting Snowball as heroic, Orwell wishes to validate the Trotsky narrative and denigrate the Stalin narrative of history that become so dominant after Trotsky was eliminated from Russia. At the same time, I think that Orwell needs to depict Snowball as heroic in the construction of the story. Outside of Snowball and maybe Old Major, leadership takes a fairly rough beating in Animal Farm. Jones and later Napoleon and Squealer as well as all of the pigs and dogs show leadership to be a corrupting force. Snowball is the only real leader who does not allow power to corrupt him and his belief in the relationship. His faith in the revolution is still evident even when he has power. In this, Orwell wishes to show that good leadership in the position of political power is evident and does not have to be corrupting. While the examples of it being a force of corruption are evident, it does not have to be the sole trait of power. In this, Snowball operates as a potential hope for redemption in so much that is wrong in the dissemination of power in the novel.