It is a known fact that criminals such as serial killers tend to return to the crime scene. As you recall that was the case for Panzer who was pretending to help out. So if Heller still wants to be authentic in his serial killer profiling, Partridge is indeed the prime suspect to the viewers since he is always there for Red John's crime scenes.
However, since Heller has already given us a taste of a typical serial killer with Panzer, I would suspect anyone else just from a story line perspective to make it more interesting. Of course Heller also said we will be disappointed when we discover who Red John is so in that case Partridge it is.

However, some of the others on the list could have been at the crime scenes but we just not see them except for Kirkland who was there for Lorelei.
Many of you have already dismissed Kirkland for XYZ reasons because he did not kill Lennon the typical Red John's way, he did not shook hand with PJ at the right time or he did not enter PJ research scene at the right time. 
If RJ is that clever, wouldn't he make sure that he has alibis during all these times. "cannot be me I only found out then... cannot be me, I did not have contact with Lorelei during that time" 

If Heller is using the serial killer profiling of the 'returning to the crime scene' and let us investigate the clues those  two are my main suspects.