My bet is Thomas Macallister; he is a sheriff, he moves in the country, he has a lot of freedom to do what he wants. He has no living family.  His blood lust can be easily seen: as well as his primitive strength necessary to be red john. What it is not evident its his intelligence (but as Jane recognizes, he looks more dumb than what really is).  He is an old fox knows how to move without being seen (see Van Pelt season 1 episode 2), he needs to be in total control of the environment around him in order to reveal himself(he admires Bach rigor in music). Brett Stiles knows him as it should be since they have similar ages(probably they were kind of buddies at the Ellison farm).