I am currently still catching up with the Mentalist and I'm watching season 5 episode 2 next. I agree with those who say Red John may have been written as one person in season 1 but that his story was changed as popularity grew.  This makes for a lot of loose ends on the show but I'm going to take another stab at this.

My main suspect at this stage is Bret Stiles.  He seems like a major person of interest as he is in the show time and again.  On the other hand I feel like he is too obvious.  The writers would like to shock the viewers eventually so why use Bret.  I can't decide whether to accuse him of being Red John. In one episode his tea cup is seen on his left, meaning he may drink tea like RJ, left handed. He is powerful, influencial, rich, and has a string of disciples who will do anything for him.  Thus he fits the bill.  PJ called him Red John and he says he knows RJ better than PJ knows RJ.

I want to look at some other things that are suspicious in my next post.