With the next episode revolving around Haffner, I thought this was the right time to put in a theory. 
-From the physical description from Rosalind, Haffner fits the best.
-He is the right age (5x13) 
-In the episode Haffner first appears in Lisbon tells Jane that "He was an FBI agent in the Organized Crime Division, and was an expert in wiretaps and creative surveillance techniques". In the season 1 finale we see that Red John has set up some surveillance to watch Jane and I don't think any of the other suspects have expertise in surveillance. I'm pretty sure RJ has set up other types in other episodes but I would need to look back.
-He works for Visualize and he used to work for the FBI both have some type of connection to RJ.
-Lisbon already thinks that he is Red John.
-This might not mean much but Haffner's first episode was wrote by Bruno Heller and it shown Jane shaking his hand.
-I thought it was strange how he asked Cho if he was smarter than Jane.
-He's an excellent whistler.
-If Haffner was Red John it would explain why he left Lisbon alive.

If Haffner isn't Red John I would say that it's Bertram, but with 5 suspects left it was always going to be tough to decide. I would say there is zero chance that its Reede Smith though.