Personally, I think that Red John is Robert Kirkland. Here are my reasons why:
-When Patrick Jane and Robert Kirkland first met, Jane did not know Kirkland but Kirkland said that he knew Jane.
Here's their first meeting:
-Robert Kirkland had some men sneak into Patrick Jane's locked room in the CBI and take photos of all of Jane's information about Red John.
Here are all of the scenes involved with the break in event:
-Robert Kirkland killed Jason Lennon, a man who worked for Red John, before Patrick Jane could get any information from him.
Here's the scene where Kirkland kills Jason Lennon:
-Robert Kirkland is a Homeland Security Agent and can therefore get any information and he must also have lots of friends in all sorts of fields, positions, etc.
-Robert Kirkland is very mysterious, secretive, and sneaky in all of his scenes.
It seems too obvious to be Robert Kirkland but, out of the seven suspects, he seems to be the most capable of being Red John and all of his scenes definitely depict him that way. I guess we'll just have to wait and see.