This is more a summary of my top suspects 

-Brett Partridge: There is a lot of good evidence supporting him as RJ. Some may argue he is too young, but that is not the case. I won't rehash what evidence has already been said but I will add that in the pilot when Jane is describing why it isn't RJ, Partridge looks very uncomfortable and we assume it's because he is getting called out by Jane. But if he is in fact RJ his discomfort can be explained in that he is surprised how well Jane understands him. Thus this moment is the start of RJ's liking toward Jane.

-Redshirt guy- The guy in the finale of S3.

-CBI Ron He's in the background, always there, we don't know much about him. Very well could be RJ.

-Kristina Frye- She is my oldest suspect so I have a hard time dropping her. While everything is making it almost certain RJ is a man, if the writer's throw in some kind of twist that could explain RJ being a woman, then it's K Frye for sure.

My other explanation would be
-RJ is 2 people (not an entire organization: This would clarify the rape of Lorelei's sister (I don't believe RJ would rape a victim)
-The writer's will throw in some curve about RJ when he is revealed that will be impossible for any of us to have predicted (like RJ is some long lost brother)

Who RJ is NOT
-Robert Kirkland
-Roy Haffner

I don't know why so many theories point to these guys just because they were made to look like suspects in recent episodes. The Mentalist never has the obvious person as the culprit. For example, S3 finale, of the 5 suspects, LaRoche and Bertram were the ones made to look like RJ's man, but neither were. Also see every other episode of The Mentalist ever for proof of this.