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.