I think most likely its someone we never heard of but if I had to take 3 of the suspects I would go

1. Arthur Leigh Allen, this really all hangs on Cheney's story either he was honest or he was just pissed with Allen, Im more leaning to the latter but it still leaves a high chance
2. The Unabomber. I know a lot of people laugh at this suspect and so did I until looking into it more, there is a lot that would strangely add up with him although some stuff that dont fit as well.
3. it wasnt one killer but unconnected crimes and/or a hoax, at first I thought this had a good chance but after looking at it more I'm much more uncertain, still it wouldnt completely shock me if turned out to be.

I dont think it was Ross O Sullivan simply cause he was like 6 foot 2/3, when witness descriptions were somewhere between 5 foot 8 - 6 feet, I think Allen is at the upper limits that could get away at 6 foot but dont find it believable that you could mistake 6ft3 person as 5ft8 espiceally in the 60s.