I`ve changed my mind again. I`ve decided to create a theory based on how Red John is (how I think he is based on the evidences), and I think this will make sense for you, too. 
We all know that Red John`s major satisfaction is when he murders his victims, when he sees the life getting out of them. Still, I think that now Red John also has another big favorite hobby: seeing Jane feeling destroyed by RJ`s murders, and by the fact the he still is there, watching him with every step he makes. So the question that comes up is: Who is the person that is almost always there when Red John kills someone? Who is carefully analyzing the crime scene, waiting for Jane to come? Right, Brett Partridge. Is him, guys, I have no doubt. His voice, his look, his way to be, the way he looks at Jane, the way he says his name. This guy is sick. 
In a show like this, any detail counts. So have you noticed that Brett Partridge has change his look A LOT? I mean, in the first season he looked like a loser, and now he has a cool hairstyle, and he dresses in a different way, more stylish. Could this be a clue? Yes. As Red John becomes more and more shinny, so does his alter ego, Brett Partridge. I`m not saying he has multiple personalities, I am sure that he is 100% aware of his actions, but I think Brett Partridge`s evolution is determined by Red John`s.
If you think about it, in the first season neither Brett or Jane couldn`t stand the idea of being in the same room, and now, in the fifth season, they seem more..."friendly". I`m pointing this because we know that RJ sees Jane as and "old comrade, rather than an enemy" (Lorelei) and I can see that Brett`s behaviour with Jane has changed. If in the first season he accuses Jane of messing around and being a clown, in the last episode of season 5 when Jane strangely asks him if he thinks the murder was made by RJ or not, he answwers "I am just guessing. You are the expert". What a change of attitude, huh?
So this is it. All I have said before, plus the most important clue - his voice, which is exactely the voice RJ puts when talking to Jane, lead me to the conclusion that he is the one. Brett Partridge is Red John. He has to be him.
Fun fact: I have a friend who doesn`r watch the show, so she has no idea what it is about.I`ve decided to make an experiment, so I asked her to listen Red John speaking. Then I asked her to listen to all the seven suspects speaking. When Brett Partridge voice started to play she told me "I`ve listened to this one before. Is the same voice as Red John`s".