The data isn't conclusive enough to lead us to one suspect and rule out the others. This is a fact we have to admit before we do any further analysis. The theories I've read so far here and elsewhere are mostly wishful thinking. Currently the question "who is Red John?" can not have a strictly rational answer. So the appropriate question to me is more like "who can we expect to be Red John?". Despite the data being inconclusive, we can still take advantage of whatever we've seen so far in the show. Anything beyond that is mere speculation. Rosalind Harker's description of Roy(Red John) is a good start. Here it is:

Just under six feet tall,
Not muscular but not soft either,
Short straight hair,
A gentle voice,
Rough strong hands,

Two other facts we can rely on:
Red John's apparent connection to Visualize(The Red Barn).
And the fact that he's a charmer and can manipulate women pretty easily.

So lets apply these to the suspects,

Gale Bertram(Michael Gaston ): Height 6' 3"
Thomas McAllister(Xander Berkeley): Height 6'
Reede Smith(Drew Powell): He's good enough for "just under six feet tall".
Bret Stiles(Malcolm McDowell): Height 5' 8"
Bob Kirkland(Kevin Corrigan): Height 5' 9"
Bret Partridge(Jack Plotnick): Height 5' 11"
Ray Haffner(Reed Diamond): Height 6'


Bertram's height and baldness definitely rule him out.
McAllister is bald, so there is no hair to be short and straight! He's out too.

These two are definitely out. They fail to fit Harker's description no matter how you interpret it. To me Smith has a soft body an not a gentle voice. And It doesn't look like Stiles has short straight hair or rough strong hands. But then again some of you might find Smith's voice gentle, or Stiles hair short and straight. 

That leaves me with three suspects: Kirkland, Partridge, and Haffner. Kirkland is just too obvious, the limousine scence, the only handshake with Jane everyone could recall, pursuing Lorelei, Killing a witness, breaking into Jane's room, and being creepy and Red John'ish in general. I just can't buy it. Partridge has been a suspect since the very beginning of the show, he fits Harker's profile but not the two facts I presented. Haffner looks like a charmer, he's a Visualize member, he fits Harker's profile perfectly, and most people don't expect him to be Red John. He's my choice.