My opinion is that Brett partridge is Red John because he stands out too much.

His character has been intertwined with RJ from the first episode of the show. He was put there before most of the script was written, they wanted RJ to be in the first episode, even before they knew how his character would play out. The producers just needed a face at the start of the series, so that by the end of the show, when he is caught, RJ's story would complete a full circle and have a beginning too, not just an end. It's highly unlikely they will chose someone else.

In the list of the seven suspects he stand out by being by far the youngest there, also RJ nominates him last on the cd. I don't think a serial killer like RJ would put himself randomly in the middle of list like some no-name, he would put himself either first or last.

Another thing that I think is a clue is that he always tries to be the buffoon, the joker of the room, trying to grab Jane's attention and always mocking him.
 In the first appearance, in the fake RJ murder, he takes control of the room and quickly proceeds to explain how RJ killed them, "classic RJ smiley face" and declares himself "spooked to finally see one(smiley face) in the flesh", although he obviously knows it wasn't him, he's just happy to mock Jane in plane sight.
In the second murder, when it really was him, he states the opposite, he says it's not RJ. It's not an attempt to lead Jane away from the truth, it would be a very thin attempt if it was, he just likes to mess with Jane by always taking a different opinion. 
In the old lady incineration episode he enjoys making Jane sick by being overly macabre and giving disgusting details because he knows Jane doesn't have the stomach for that sort of things.

Also, when he appears on the two RJ murder investigation, the camera always films him with the :-) showing behind his shoulder whenever he talks to Jane. You can't get anymore symbolic than that. Or can you? The first time we see Brett Partridge is when he explains the path the supposedly RJ murderer took in the house. He walks through the door into the room using himself to describe the movement of the attacker. His first line is "Red John enters here".