I believe Red John to be Robert Kirkland for one subtle reason. It was hinted in the 100th episode that Kirkland was Red John because of his one line "Thank You", however if The Mentalist has taught us anything it is that Bruno Heller loves a good twist. So in the episode where you find out Red John worked at the visualize farm it concluded with Jane saying he remembers the name of every single person who's hand he's shook having missed one or two names. These one or two names is HUGE. Go back and watch the first time Kirkland meets Patrick Jane here http://www.youtube.com/watch?v=jtlMX_Y6EvI. Jane never gets the name of the man who's hand he just shook. It is just assumed by the audience Jane remembers this man. I believe that this is one of the two or three people who Jane will skip out on when retracing his steps over all the people he has met because he doesn't know who this person is.
There are many more obvious reasons why it could be Kirkland such as the voice or build or he's the right age and would have right connections but with the Mentalist it never is obvious.
