Okay i am almost certain Brett Partridge is Red John. 
Simple Explanation: The poem Red John refers to is "The Tyger" by William Blake.   Bruno Heller is a screenwriter so will use his mind and set everything out perfectly. William Blake was famous for drawing Partridges. Brett Partridge.

Okay: Detailed Explanation:

Red John 7 List of Suspects:
Bret Stiles
Gale Bertram
Ray Haffner
Reede Smith
Robert Kirkland
Sheriff Thomas McAllister
Brett Partridge

Red John Clues:
Red John is most likely a man
He appears to be a Caucasian, and is a white person as described by Haley
Red John has an average height: Rosalind described him as "just under 6 feet tall"
Red John has short straight hair (as described by Rosalind Harker)
Red John either has or puts on an odd, high pitched and soft voice
Red John should be older than 30 years (since his murders date back to the 90's)

Red John has a short straight hair, that means that Gale Bertram is not red john.He does not have short straight hair.

Red John has an average height, he should be just under 6 feet tall. Ray Haffner and Reede Smith is too tall. Haffner is 6ft1 and Reede Smith is 6ft2.

Now we are left with Stiles,Kirkland,McAllister and Partridge.

Red John has odd, high pitched and soft voice. Brett Stiles and Sheriff McAllister doesnt have soft voice.

With proven facts,we are left with Brett Partridge and Robert Kirkland.

Robert Kirkland would be too obvious to be Red John as he was introduced in Season 5 and was a red hearring. Bruno Heller also states we may of seen Red John before Season 3. Brett Partridge has been there since the beginning.