I've only read about 15 theories, so I don't know if someone else has the same opinion, but here's one of my guesses:
I believe the voice plays an important part in RJ's identity. Clearly he has an american accent (see the part with "Tiger, tiger, burning bright [...]"), so I guess Bret Stiles can easily be ruled out. As well as, maybe, the sheriff. RJ's voice is natural, there's no sign of (major) voice alteration on purpose. So, presuming that he indeed is one of the six remaining suspects, based on voice recognition I'd say he's either Bob Kirkland or Ray Haffner. Of course, there are obviously several other aspects to be analyzed...