In the novel Animal Farm by George Orwell, it at first seems as if Snowball is the best writer.

Then Snowball (for it was Snowball who was best at writing) ...

He is the one who alters the name Manor Farm to Animal Farm and writes it on the gate. He also writes the initial commandments. Furthermore, Snowball institutes classes in reading and writing for all the animals except for the pigs who can read and write perfectly. The text, however, suggests that Minimus the poet is the best writer, as the following quote from chapter 5 suggests:

Minimus, who had a remarkable gift for composing songs and poems ...

Also, Minimus is the one who, in chapter 7, composes a new anthem to replace Beasts of England. The new anthem begins:

Animal Farm, Animal Farm,
Never through me shalt thou come to harm!

He later, in chapter 8, writes a eulogistic poem entitled Comrade Napoleon in which he expresses the general feeling on the farm and sings the praises of their leader. 

Squealer cannot be considered in this regard, for all he does is alter the original commandments that Snowball has already written on the barn wall.

I think that you can find the answer to this towards the end of Chapter 5.  The answer is, I think, a pig named Minimus.  We don't see him very much, but he is the best writer.  We are told at the end of Chapter 5 that Minimus had a "remarkable gift" for coming up with things like poems and songs.  To me, that indicates that he must be the best writer on the farm.

Eventually, Minimus composes a new song to replace "Beasts of England."  He also composes poems in honor of Napoleon.