My suspect is Partridge. My main reason is that we're talking about a crime story, and in a crime story the classic way is to introduce the culprit early (he appeared in the Pilot first), then move him aside to let the spectators forget about him, thus in the end he can show up accompanied by massive surprise.
So in our case the goal is to leave us shocked by the identity of RJ, but that cannot be accomplished if we're given the list of the final seven (if we can believe what Heller said). If we do, I think that narrowing the list is only a way to confuse us, not to make our job easier, since RJ is one of seven people, not three. If we don't, then RJ can be Ron, the CBI agent or Joe, the janitor as well...

Oh, and we haven't seen Partridge's body yet.

I might be terribly wrong, but I'm pretty sure I would be embittered if RJ turned out to be e.g. Bertram or McAllister.