Has to be Bret Stiles. First of all, Red John is not that young (they said in the first crime he committed in that Barn from episode 5.13 more than 20 years ago). So, though it would've been plausible, that ruled Brett Partridge out for me (plus he is a big fan of Red John and by what we have knows RJ doesn't have the geek enthusiastic profile, and it would be very masturbatory of him to investigate and study his own crimes).
Other suspects, like Kirkland for example, are definitely connected to him but seem to lack the cunning intelligence of a mastermind criminal.
From the Red Barn episode we've also learned that Red John had a connection to Visualize, which is now led by Bret Stiles.
Bret Stils also in more than one situation shown that he is as or more intelligent than Jane. When Jane accused him (I can't remember the episode), he managed to get away and had a plan of his own. Jane (kinda) trusts him and, even if he doesn't trust him fully, he's definitely one of the suspects he respects the most (and Lorelei said they were alike).
So, he has the right age, the right background, the right connections, the right brain and the right posture.
Plus, and this may sound stupid but it's a big plus, it's MALCOLM MCDOWELL which is one of the greatest villains ever and thus being a perfect cast for this great evil mastermind.
Nuff said.
