I'm not going to hazard a guess, but offer a few points that people may have not thought too deeply about.

Bob Kirkland works for homeland security and, as witnessed in one episode, is in league with Bertram and member/s of the FBI. This leads me to believe that Red John is not just a serial killer, but presents an issue if he is caught by the wrong people, including CBI, and most certainly Jane. Red Johns resources and information also hint that he could have more information that cannot be revealed. I believe Bertram and Kirkland can be ruled out, and are acting as a third investing party for their own ends. Also, there has to be more to Red John knowing Janes happy memory, such as knowing him, or just of him, before the events leading to the start of the show took place. In what capacity I can't be sure, but Heller is very precise in his planning and I don't think these loose ends just happened by chance. I therefore think that season sixes main focus will be on the full backstory of Red Johns connection to Jane, as well as Homeland securities invested interest in him. As Heller said that anyone as Red John is going to be a disappointment I think he plans to offset that by having a great and much deeper backstory to Red John, and his connection to Patrick Jane.

If I was pushed for a guess I would go for Sheriff mcallister. I won't list any evidence to support this, Ii just feel that if he were any of the organisations (visualise/Haffner FBI/Smith CBI/Partridge) subordinates they wouldn't be able to act as freely as they do, and if it were the leader of the organisation (visualise/Styles CBI/Bertam Homeland Security/Kirkland) then it would be too easy to explain away. Also, he is the only one out of the seven that isn't in a high position of power. He is Sheriff, but I don't put that in the same league as the rest.