So we know a few things about Red John.

1) According to one thing I read, it won't be until November 24th that we actually know the name of the real RJ.

2) RJ was "young" back in 1988.

3) He at least had a full head of hair.

4) He's 5'11"

5) Muscular build.

6) Left Handed

7) Afraid of something

8) Fans will be disappointed by who he is.

9) Can whistle.

So here is why I think it is McCalister:

1) His character's stated age is younger than the actor who plays him which I find odd.  He is listed as being born in 62 or 63.  That would make him 25 or 26 in 1988 ... young enough to be the guy.

2) Right height, build, handedness, whistler.

3) He was introduced the earliest of all of the suspects on the list (season 1, ep 2). Given that RJ was the focus from early on (at least PJ's motivation), it seems likely he would be a character since early on.

4) Only one of the suspects who seems to surprise PJ.

5) Straight hair even if it has thinned out lately.

I had previously thought it might be Smith, but McAllister makes more sense.