Ive been a frequent visitor to this site, especially over the last few months and its time for my prediction as Red John maybe revealed this weekend.
This is based off one of the 7 suspects in RJ.
After a lot of thought, I'm nowhere near 100% sure on this I'm going to have to settle on Ray Haffner.
A lot of the clues i can't seem to figure out, "He is ma (r/n)"
I feel, if anything the tattoo's rule Bertram, McAllister and Smith out.
I'll explain why him and why not the other suspects.

Reede Smith
Basically if it was him I'd cry. He hasn't been around long enough. He doesn't fit any of the RJ descriptions we've had over the years. (Not tall and this)
Also admits he's a lower level guy in Tyger Tyger.
Most obvious is that Lorelei Martins would go near him :)

Bret Stiles
Basically too old and doesn't fit descriptions. But I do think he's involved with RJ which I'll explain futher down.

Bob Kirkland
Only been around since season 5!!!!! He fits the physical descriptions we've had in the past, but I guess their can't only be 1 person matching RJ description from the 7. seen people say it's him or his twin borther. I think the twin brother was a legit reason he was hunting RJ. He asked the fake suspects (Thinking they were RJ) if they had seen him (twin brother) before.
Twin brother just another BS red herring from Heller i think.

Brett Partridge
I wouldn't mind at all if he turned out to be RJ. He's been around since the pilot and his first line ever was "Red John enters here". He's at every RJ crime scene and acts strange. The fact he's dead and it seems a little farfetched, even in this show, that he faked his death. He fits the description as good as anyone on the list honestly but surely way too young to have been at the Visualize Farm in 1988, couldn't be older than 10-15 back then surely.

Tom McAllister
He's been around since episode 2. Definitely creepy enough but just can't see him as RJ. Doesn't seem smart enough even if he is hiding it. He seems to like Jane too. Has the tattoo so a mid level tyger tyger guy i assume.

Gale Bertram

I think the tattoo rules him out and he's just a higher member of the Tyger, Tyger group. Doesn't fit descriptions as he's well over 6-foot and we've seen RJ from behind and he has hair. He's looked like a likely suspect for a few seasons now and can't see it being that obvious and surely Jane would have figured it out.

Ray Haffner
Okay, here's why i'm not 100% sure. He's only been on the show since the start of season 4. He's been accused multiple times of being RJ and reacted genuinely angry. He's been my top suspect since mid-season 5.

I think Ray Haffner is Red John. He's a member of visualize which explains why Bret Stiles has so much knowledge about Red John activities. As he's in visualize he could have been at the Ellison Farm in 1988 and would have been (as pointed out in the very same episode) 21 years old which fits the description that "a kid" painted the smiley face. He fits the recent descriptions, being tall and slim, not bad looking. He looked suspect as he was showing Jane if he had the tattoo. He acted angrily when Lisbon said he thought he could be Red John in last weeks episode instead of just saying no. This may have been done to rule him out as a suspect for Lisbon. He has a phobia of spiders which makes sense because in Sophie Millers notes she said he may have a phobia of something other than heights.

Summary
There's a few reasons why all these people shouldn't be Red John but Ray Haffner has the fewest I can find and has some good evidence supporting the fact he is.
If is someone not on the list eg. Walter Mashburn, Minelli or Ron lol I won't be too pleased unless they have the most complicated back-story of all time.

All I hope for is an epic reveal which links all clues we've got over the 6 seasons.