I think something was given away in the last episode...
Kirkland talked about a twin brother...could it be that at one point his twin impersonated him leading to Jane into
thinking Kirkland was a suspect?
Kirkland's brother (when he disappeared) could have left to join Visualize and at that time became Red John.
No one will suspect it because Kirkland is not one suspect but two; with Bob now dead who would think twice...

Here is why Reede Smith is Red John.  And if not, he is at least one of the top 3 of the remaining suspects:

Aside from all of the "what's in a name theories," which may or may not be relevant, there was something I noticed in tonight's episode that caught my eye.  When Reede was invited early on in the episode to the private meet-up in the park by Kirkland, Kirkland slyly used the "tyger, tyger" code phrase to test whether Smith was part of the organization.  If you'll notice, although Kirkland did it mid-speech, and in a pretty convincing manner, Reede Smith didn't even blink and acted as if he had no idea what "tyger, tyger" meant.

But, at the end of the episode, after he kills Kirkland, he turns to the prisoner transport driver, whom it would appear he had never met, and instructs him how to cover it up.  Then he uses the phrase "tyger, tyger" to the driver, a person he seemingly had never met, but must've known was indeed a part of the organization (although the driver apparently didn't now him, otherwise why the need for the code words).

So, Reede Smith is either Red John, or he is so high up in the organization that he knows everyone who is and isn't a part of it.  He knew Kirkland wasn't a part of it, even though Kirkland used the right code in at least as convincing manner as he and the driver did later.  And he knew the paddy-wagon driver was, presumably without the driver ever having met Smith.

It was as if the "tyger tyger" code was irrelevant to Smith, who knows all who are a part of the organization whether they use the code or not.  Yet, he uses it to rally his troops, and confirm to those who may have never met him (i.e. the driver) when needed.

Bertram - It appeared obvious in the meeting between Smith, Bertram, and McCalister a while back, that Bertram is subordinate in the food chain to Smith, reminding Bertram of his job to keep an eye on Jane.

McCalister - who was completely aloof in the same meeting, and appeared ignorant of Jane's investigation into Red John, was either playing it so cool as to not reveal his identity to the others in the room, or he is legitimately not Red John.  The other two in the room seem to ignore him.  However, he is also the most relaxed, almost careless in his posture in the meeting.  While the other two men are engaged in Jane's words as if trying to protect themselves.  

Stiles & Haffner - are the other two possibilities, but I think they are too obvious as choices.  Too naturally sinister.  Smith is a much better "pay off" for the show than either of these two guys.  I think his character will be more and more developed the longer this season goes on too, which will make it even better.

While McCallister could've been a good choice, I don't think his character has been developed enough in the story to make him a worthy Red John.  Based on the clues from last night's episode, and the way Reede Smith's character has been annoyingly sprinkled in to the plot over time, I think Reede Smith as Red John is the most plausible suspect.