In accordance with the title of this thread, I'd have to say the hardest chainsaw to find that I know of would be the Charles Wolf gas-powered two-man sawing machine. I've heard varying estimates on how many were made but the most consistent number I've gotten so far is around 16-18.
Another extremely rare chainsaw is the Burnett Model 'C'.
Another extremely rare chainsaw is the Burnett Model 'C'.