"Finding Red John is like finding waldo in a children's book." 

I watched the Red Mile today on TV and after watching it I noticed something shocking. Dr. Steiner has a slouched posture, and when Red John saves Jane in the end of season 2, he also has a somewhat slouched posture.
Dr. Steiner matches Red John physically in that aspect.

Also, Dr. Steiner is fairly old. From the previous episode, "The Red Barn", we have found out that Red John started his killing spree much earlier than we thought. This means that Red John is fairly old, and this matches Dr. Steiner since he's old as well.

Moving on, we know Red John is a mastermind and can be deceptive. Dr. Steiner seems to be clueless through out the whole episode, but randomly in the episode he shows some great understanding of the mind and it's ability to cause hallucinations (signs of Dr. Steiner being skilled in hypnosis maybe? ). Regardless, Dr. Steiner definitely could be Red John.

Lastly, Dr. Steiner and Jane seem to like each other and Jane plays tricks on him and Steiner finds them fascinating, like a kid watching a Disney movie. He is very good at hiding his true identity, his Red John identity. Also, Dr. Steiner has Patrick watch him kill himself with pills. Maybe he did this so Patrick would never suspect him to be Red John.

Also, Dr. Steiner dies (fakes his death I assume) a few episodes before the epic season 3 finale. So now Red John is off the radar, and Patrick will never know. Here we are in season 5 thinking CBI Ron is red john or brett stiles, when clearly they can't be red john since they're too obvious, etc.

If Dr. Steiner isn't Red John, then I'd have to place my money on Walter Mashburn.

A few reasons to think why:

Dr. Steiner, although "dead", should be a prime suspect for Red John. Sure you'd have to get around the fact that Patrick witnessed him dying, and yes there are a lot of theories about his death being faked, so moving forward with these thoughts we will have to at least assume  that is a possibility. 

Dr. Steiner fits the profile of what Red John is Like physically, as well as his proven mental capacity. Of course if he really was he may not show just how smart his around Patrick so as not to create to much suspicion. 

He also was not included on the List of suspects that Patrick wrote down after Lorelei made the comment about her being surprised that Red John and Patrick didn't become friends after they had shaken hands. 

For anyone curious Patrick's current suspect list is as follows:
Ellis Mars
Dean Harken
Jason Cooper
Walter Mashburn
Vint Molinari
Dr. Linus Wagner
Virgil Minnelli (who is my second choice personally for being Red John as he also fits all the other pieces)
Dr. Towlen Morning
Osvaldo Ardiles

That being said clearly Steiner is not present on this list which gives him a perfect cover but only time will tell. 

The biggest thing that led me to Steiner however was in season 3 during the episode in which he dies. After he makes the phone call about having the body him and Jane start talking. Steiner brings up Lisbon and asks Jane if they will let her in on the secret. Jane tells him in time, which (I don't remember the exact phrase of the comments) Steiner makes another remark and then makes this comment:

"She reminds me of... well never mind"

No further comments happen after that. 

It may be considered a stretch, but since it happens as a passing comment and is never brought back up, I think it's fair to say that it as a slip of the tongue with a reference to Jane's wife. I understand the Red John would have never really have met Jane's wife, but I think that there is a possibility that this is true, especially considering some other comments about Jane and Lisbon in recent episodes. For instance Lorelei's 

"I think you're just a little bit in love with Theresa Lisbon".

This is a working theory and I'm not sold on the idea, however after rewatching season 3 and seeing this episode makes me wonder. 

If all else fails here my vote is Virgil Minnelli

Good Luck Investigating

What if Dr. Steiner took a drug that made his body seem dead to most people, but was just really sedated.

He would then have one of his followers, possibly in the coriner's office, declare him dead.

I have been studying this case very closely and Dr. Steiner - He's the guy! He is not dead but had Jane think that he assisted him in his suicide. Oh! How clever - but Jane will catch him in the end. Inspector Clouseau agrees with me.

I have a strong feeling that it is Dr Steiner

From the knowledge that we have of Red John, he fits all the requirements.
He also made Jane witness his death, therefore he is now believed to be dead. Yet Jane did not see him take the medicine. Dr Steiner said he didn't want to end up on an autopsy table, obviously because then they would come to some conclusions and link him to RJ. 

Red John is Dr. Steiner...I had thought it was him for a while, but wasn't sure until he asked Jane to witness his death. It has to be an older male, of average but not athletic size, who came to the C.B.I. after Jane did. It has to be someone higher than average intelligence with medical knowledge. It has to be someone close enough to Jane to keep track of him, but not close enough that Jane could accidentally pick up on who he really is. It has to be someone noone would ever suspect, and how could anyone suspect the man that Patrick Jane himself witnessed dying? In the episode that Dr. Steiner dies, he makes a point to tell Jane that he doesn't want to be on the autopsy table. Of course not, that would actually kill him. They never actually showed him taking pills to overdose, so noone knows what he actually took. So being a doctor he would know how to fake his own death. I could continue, but let's see the theory so far get broke down first....