After watching all the episodes and reading the theories, this is what I have come up with.

Walter Mashburn- seems like a good candidate. Rich, powerful and charismatic. However, when Lorelei said "I only wonder why the two of you didn't become life-long friends the moment you shook hands.", I think we can safely eliminate him. As we saw in the previous episodes, they got along very well and we can even say that they became friends. Therefore, Walter Mashburn isn't Red John.

Brett Stiles- seemed like a good candidate until Rosalind Harker's description. On the other hand, I strongly believe that he does have some information on Red John as he does give PJ some information about him etc. He will be a key figure in the hunt for Red John.

Brett Partridge- favourite RJ prediction on this site. I have to admit, he does seem like a good candidate. However, he doesn't look to be the right age to be RJ. Also, Brett gives me the impression of being a bit weird but not RJ weird.

CBI Ron- Does seem like a 'Waldo' character just like Heller said but hasn't spoken a line since the start of the show if I recall correctly. Also when Rosalind describes RJ, she doesn't mention facial hair and says he is quite slim.

Bob Kirkland- Contrary to the popular opinion, I think he is a positive character. As we see from the last episode, he wants to find about RJ. My personal guess would be that he is very similar character to PJ. For example Lorelei/Miranda was somehow related to him and he wants to avenge her death (can be someone else). You can look at him like a Snape character (from Harry Potter), first the viewers see him as a negative character but as the story progresses we find out that he is actually very positive. Same as Snap I think Kirkland is gonna be killed by RJ (or someone else).

Dean Harken- 'flavour of the week', he seems as a freaky person. Moreover, has red hair and talks about his powerful friends- "I'll call my friends at the FBI, get them to take over from you. In fact, I will do that.". Seems like a decent candidate for RJ.

Gale Bertram/Virgil Minelli- both seem like nice guys (especially Virgil) and don't fit into Rosalind's description. Bertram is working with Kirkland so in my mind that would eliminate him. However, he did recite 'Tyger Tyger' to Lisbon meaning that he knows RJ (without knowing it) and heard it from him.

Ray Haffner- doesn't seems clever enough but does fit into the "when you shook hands" thing. Still don't think its him.