Michael Nunn to the body. Chris Eubank Sr delivered the best one I've ever seen during his fight with Gary Stretch.
Foreman has not been mentioned. Very few guys have the balance to throw this punch. Very very effective if it is part of a guy's arsenal.
I agree with Marvin Johnson and George Foreman. Another guy that comes to mind if you count it is Razor Ruddock's half hook half upercut he used to throw.
Tyson`s rear uppercut was more fearsome than his left, he used to follow up his right hook to the body with his rear uppercut awesome.