This is a tricky question. If we are looking in terms of sheer writing ability in terms of the gift of prose, I would side with Snowball. His construction of the commandments of Animalism and his speech in convincing the animals of the need to build the windmill are both very persuasive. They demonstrate both his excellent grasp of language as well as his ability to sway other animals towards the ends of animalism and the revolution, in general. Snowball's use of rhetoric is fairly unparalleled. In the battle of Cowshed, he proves to be a very inspiring commander with his calls to battle, a reflection of his writing ability. A very close second would have to be Old Major. His speech in the first chapter is rousing. It is apparent that Old Major has some excellent skills in being able to convince an audience of his visions and ideas. I go with Snowball over him because Old Major dies rather quickly and there is little idea of how his skills would have transferred to the actual possession of power. Snowball demonstrates the writer's eloquence both before and after the revolution.
If we are looking at who proves to be the best in terms of most successful writer, it is Squealer. Squealer's gift of "spin" is what keeps him in business. Napoleon is not much of a writer, so his need for Squealer who is able to "put the music" to anything Napoleon does and says is what makes him the most successful writer. Squealer is so good at writing that he is able to rewrite history without the animals knowing or at least being scared enough to not say a word about it. Squealer rewrites all of the commandments of Animal Farm/ Manor Farm as the pigs consolidate their power. At the same time, his ability to use language as a way to confuse show a real gift with words. His reconception of language to become a tool of power is something that even Old Major and Snowball could not do. I think that in this, Squealer has to be seen as the best writer.