I believe Red John to be Sheriff Thomas McAllister. 

I rewatched season 1 this weekend, and in Episode 2, Sheriff McAllister "loses" 6 times in a row to Patrick at rock, paper, scissors. I believe it wasn't Patrick that was playing the Sheriff, but rather the Sheriff was losing on purpose to Patrick and deceiving him all the while by making him believe that he bought into PJ's psychic charms. This is obviously a recurring motif of the RJ vs PJ contest, as Red John always seems one step ahead and in Patrick's head as was revealed in the latest episode. This maybe an over-analysis of a singular incident, but the Sheriff was looking Patrick directly in the eye during rock, paper, scissors, and if you've ever played the game, you always watch your hand or your opponent's hand, which gives rise to the possibility that RJ was actually playing Patrick's game all along.

Furthermore, at the end of Episode 2, Sheriff grabs Van Pelt and there is a brief moment of shock as Patrick and Lisbon are watching the surveillance inside of a motel room, where they then proceed to dismiss his involvement in the crime for no reason whatsoever. The Sheriff forcefully grabs Van Pelt and if Rigsby hadn't arrived in time, the Sheriff would have forced Grace to his car. For some reason this all disappears as soon as the Chef and his wife are caught at the end of the episode. 

He was only in one episode if I'm not mistaken, so I believe it would be a clever way to end the series with the Sheriff being RJ as he is one of the most overlooked characters in the show, evidenced by the fact that I must post this theory under the accusation of "Someone Else" :P