He has been there since the beginning: "Red John enters here."  
It's the perfect poetic justice/balance.  
I don't care if it is too "obvious."  He is so Red John creepy/smart/just socially awkward enough.
Just the way he looks at Jane; he's the under-dog that you almost can see is justified to get back at Jane for all of his "bullying" and ribbing. 
Fakes stupidity.
Under their noses -- hidden in plain sight.
Had complete control over staging his "death" scene -- motive still to be revealed. 
He's only in Red John frame episodes (beginning and ending of seasons).

All in all, whoever it is, most fans (who read this site) will be pissed and disappointed, right?  After reading so many theories, I think that the Bruno Heller is damned to disappoint at least 60% of his fan base.  

But, Brett is the only satisfying choice, in a literary sense.  The others are (read: should be) red herrings to hang evidence on.  If they are Red John it's like Big F'ing Deal, okay good job.  Some random dude that wasn't in the pilot -- added in the middle of the show, who may have been in 2 or more episodes before this season, yay! 

I still think Kristina Frye's involved -- if there is no point to all the red-headed women in this show, for symbolic meaning, then there was a very missed opportunity.  

Brett and Kristina sitting in a tree, K-I-L-L-I-N-G!