It's Sheriff McAllister. Here's my case:
1) Red John has a deep connection to the rural areas outside of the city. Yes, he has been active in the city areas but it's not his normal stomping ground - look at season one finale which introduced both a friend and a lover of RJ, both of which were in a rural area. And RJ was friends with that killer's dad, so RJ has deep roots in that area or community.
2) Rosaland mentions that RJ smells of "pine and earth" which seems to be answered because Jane thinks he was building a cabin or something...but, I think that's how he smells all the time because of where he lives and the life he leads. His friends are from small towns - and so is his past...which leads us to...
3) The barn where it all started. Visualize is always the thing people mention when discussing RJ's first murder, but I actually think where it happened is important too. Another connection to the rural areas.
4) The first time he met Jane they "shook hands" in a unique way that played up the idea of Jane being a false psychic...it's the perfect intro for the eventual villain...and could be used as a wrap around later when they meet for real.
5) Episode 2 dealt with a couple who kidnap and sexually assault young girls, just like the other couple from season 4 (I think) finale. Also, it's possible the couple from episode 2 was being encouraged by RJ, but of course they died so...who knows...
6) He's the suspect fewer people look at...why? Because not much is known about him. He doesn't really make much of an impression whenever he's on screen...which is odd since the actor who plays him is quite talented....maybe he's supposed to be bland?
7) Most of the other RJ suspects are this season in serious nail biting episodes...but not Sheriff McAllister....he's practically in a romantic comedy...
again, we don't take him that seriously...perhaps it's by design?
In summery -- Red John has connection or works in law enforcement, plus he has a lot of friends in the suburban and rural areas. Sheriff McAllister.
