I still think its Rigsby. 
Don't know why no one has ever mentioned the ring he ALWAYS wears on his right hand.  Heller said there would be clues in the titles. 
s1 36 Red Handed
s2 e8 His Red Right Hand
s4 e4 Ring Around the Rosie
Not saying its a bullet proof theory, but the ring is a prop.  He has it on all the time, and actors don't wear personal jewelry to film in.  Why would someone wear a ring on the right hand, not wedding finger unless it meant something.
Also when Rigsby was hypnotized, Jane said true colors came out.  Rigsby was violent. 
On the flashback episode, when Rigsby met Jane for the first time he seemed shocked. 
I don't like the way the show was left off either way.  Obviously I have been looking for clues, and want at least some closure, what were the clues leading up to Sheriff?  A little explanation would be good.