I put forward theory #2905 just under two days ago, wherein I argued reasons why I felt Red John was Patrick Jane, or if not him then one of the core four. Failing that, it absolutely has to be someone we were introduced to in the show's pilot episode given my reasons in my post (#2905). 

Rewatching the season 5 finale for the 5th time (yes 5th), I can't help but suspect Brett Partridge even more. I had urged everyone who read my post to pause the finale at the second Partridge turns to face Patrick with the Red John smiley face dripping on the wall behind him. I don't believe people bothered, therefore I am urging you now to look at this screencap I have taken, before it gets taken offline:

https://plus.google.com/u/0/photos/101232154949211716790/albums/5875750306743394993

Coincidence that his face matches the one behind him exactly, yet by the time he has finished turning he looks innocent and surprised at Patrick's presence?

More to come...