I believe Red John would have to a possible homosexual man (no offence to anyone). Why? Because his regular victims are females and he's killed males only when they show a possible threat to him or his identity. This might sound like a weak theory, but as a student of psychology, I'm forced to analyse the personality of the person who is Red John and why he only kills women as his "regular" victims. 

Brett Partridge does make a good suspect because of his placement in the Forensic Dept,his attitude towards Jane and the RJ case investigations, his voice, his appearence (that matches Rosalind and the little girl's description) and moreover, to support my theory, Mr.Plotnick has potrayed his role in a somewhat feminine body-language, almost homosexual.

Of course, there's also the fact that RJ is a rich, highly influential person, making Partridge innocent (except we don't really know his financial status). 

This also makes me think RJ could have something to do with Brett Stiles's cult "Visualize" and it's recurring appearance. Even though Stiles is quite old to be RJ, it could either be him or one of his disciples. 

And finally, I personally think we can suspect one more person: Mr.Amos Van Pelt (Grace's father). I thought of him because of what that FBI mole said about him when he met grace for the first time, mentioning what a great influence he was for him back in high-school.

So here's my suspect list:

1. Brett Partridge 
2. Brett Stiles or a protege of his.
3. Amos Van Pelt (just a hunch)