By combining several theories, here is my conclusion.
Red john is the seven headed dragon and the biggest head belongs to Thomas McAllister.
He is definitely strong, he afraids of birds( his phobia) and he was one step ahead of Jane.
Apart from the reamining suspects,
Kirkland's twin brother is the sixth head.
Brett Partridge is the seventh and the wounded head and I think he will reappear in the later stages.