George Orwell's Animal Farm is full of interesting characters, and the plot really lends itself to journal-writing because all characters can all write about what is happening to them and what they think about it.
The first decision to make is whether you want to write as a "good guy" or a "bad guy." If you choose the latter, obviously you will have to be one of the pigs or dogs. If you choose the former, you have many more options.
The next thing to consider is whether you want to write about someone who stays on the farm or leaves it. Mr. and Mrs. Jones, Mr. Whymper, Snowball, Mollie, and Moses are the characters who leave, so you could write all kinds of interesting things about their lives away from the farm. I am guessing your teacher would prefer that you write about one of the characters who stays on the farm, however, so that kind of omits them.
One final consideration is whether or not you want a character who more or less understands what is happening on the farm, If so, you probably want to choose Benjamin or Clover; if not, any of the others are possibilities.
In the end, your choice of character should be based on what kinds of things you want to write. If you have no preference at all about the way you want to write, then choose the character for whom you have the most feelings (whether those are good or bad feelings) and write from his or her perspective. If you want to write about the confusion the animals must have felt as the farm undergoes such dramatic changes and the animals are being lied to, pick someone like Boxer who loyally does what he is asked but is often puzzled by what is happening around him. If you want to write about propaganda and how easy it is to fool the animals on the farm, Squealer is an excellent choice. If you want to write totally dispassionately about what is happening, observing everything but without any understanding of what it means, the sheep are certainly good examples of that.
Once you decide the way you want to write about the events that happen on Manor--no Animal--no Manor Farm, you will be able to narrow your choice of character to one or two. A good model or example for journal writing (which is just sharing thoughts about things) can be found in a paragraph in which Clover ponders what has been happening on the farm:
If she could have spoken her thoughts, it would have been to say that this was not what they had aimed at when they had set themselves years ago to work for the overthrow of the human race.
In the end, you are expressing the imaginary thoughts of a character, so choose a character for whom you feel you can (and want to ) speak.