Two main suspects for me: Sheriff McAllister and Gale Bertram, in that specific order, with a lot of support from a bunch of characters we all know (Partridge, Styles, KFrye, CBI Ron, etc.) Not giving credit at all to Rosalind Harker's description of RJ since it's been demonstrated she might  be fooling us... What i take as a lead is Rebecca's reaction when she's approaching soon to poisoning her RJ at the prison and the body structure of that particular man, IMHO only similar to that of McAllister and/or Bertram... Many things have been detailed about Bertram but not enough have been said about the Sheriff so i'm leaning that might be a misleading omission for us to think RJ might be somebody else... We'll see in a few weeks... Enjoy S6!