It is Bret Stiles...yes.... the least suspected guy because of his physical characteristics...but my theory actually supprts this fact... Stiles is red john but but as he is ageing, recently the kills were done by his disciple Ray Haffner...
In episode "Red John's Rules," you hear Partridge say "Red John's been laying low for years, working through friends and proxies."
This is also confirmed by S06E06, where Stiles says that he is dying..
It was Haffner in S06E01 who killed partridge and drew smiley face on lisbon's face. We can see Haffner's hand caressing lisbon's chick while drawing smiley..