I think that Red John is Sheriff McAllister 

He is the only one besides Brett P. that appeared in the series in the first season, and Heller mentioned in one interview that he knew who RJ is going to be since season 1. 

His physical appearance is a match to the RJ description:
- He is under 6' tall 
- Not muscular but in good physical shape and strong handed 
- He appears to be middle aged
- He is calm and patent
- Good whistler 

He is a hunter and stated that he hunts -" Anything with a face is a fair game".
Patrick noticed that he moves very quietly in the episode - Red Wedding
He is friendly to Patrick and has stated multiple times that - he is always available if Patrick needs him. 

When Lorelei said to Patrick - "I don't know how you two didn't become friends the moment you shook hands." I imidietly thaught of the first time they meet. McAllister was intrigued by Patrick and you can really see his playful side when they "shook hands" - played - rock, paper, scissors.

Plus the actor that plays the Sheriff -  Xander Berkeley is a really good at playing bad guys.