First, I think there are a lot of really clever people here who are obviously dedicated fans of this show and they have given a lot of thought to solving this puzzle. It is great that this show and its talented cast and writers have generated this dedicated following. I really love The Mentalist.
The writers have done a nice job keeping this reveal plausible for multiple interesting characters. Good for them!
My guess is Haffner. Here are a few reasons (although others have provided many more):
-He is the only suspect with clear connections to both law enforcement and Visualize (allowing him to develop acolytes in both organizations)
-His law enforcement connections would put him in a position to set up Tyger, Tyger as a decoy, since that group may be a threat to him as well.
-The next episode says "Only three suspects remain" and then shows the three tattoos. But we don't know which suspects are killed in the blast. If a Tyger is among those killed in Jane's cottage, then one of the Visualize suspects could still be Red John.
-Haffner's personality is creepy and immature while being wily and clever. This fits RJ, who often taunts Patrick like a kid on the playground. "There's going to be a comeuppance," he said.
-Finally, Haffner has a thing for Teresa. If RJ wanted to destroy everything Patrick loves, he would have killed Lisbon long ago. But Haffner is intrigued by her.
That's my guess, but I understand that it is only that! I can't wait to find out.
