Well, well. How convenient that Brett Partridge shows up in the beginning of the finale. I have suspected Brett for some time now, but even I was suprised by his appearance in the finale. 
I also must admit I find Ray Hafner to be an intriguing RJ suspect, given that he was in the FBI and has ties to "Visualize."

But all evidence still leads to Brett. He has been around since the pilot and quotes classic literature just like RJ.

Brett Partridge is Red John.