There are only say...2-3 suspects I feel. 
Kirkland - Dead 
Stiles - Dying/Dead (and he's too old to fit any description anyway)
Bertram? - Way too obvious if it was him. It wouldn't fit the Red John legacy anyway. Besides, Red John isn't even supposed to have been revealed until next episode.
Reede Smith - He's in jail. I agree with Jane when he said Red John wouldn't open up like that. If he was RJ, he was almost killed by Tiger Tiger several times, and RJ wouldn't let that happen.

Partrige? In the beginning I thought he was 100% dead. Then I read some theories and thought maybe... But no. Cho saw the body, Heller said he was dead, and it just seems all too unlikely. It would be a fantastic plot twist, but eh. Very doubtful.

Leaving us with 2:
McAllister
Haffner

I don't think there is any other way it is not either one of these two. I choose Haffner, but its up to you guys :)