I sometimes post without reading any of the prior posts. This time I think I may have missed something..I thought the thread meant "what was the most important punch in boxing" literally, that's why I posted "the jab".
That's how I see it, too. And there we have Fitz' solar plexus punch on Corbett, Jeffries' left hook on Fitz, Willard's right on Johnson and, especially, Marciano's right hand to Walcott's jaw. Of course, Robinson's terrific left hook on Fulmer belongs here as well. I would say that the left hook Cooper landed on Clay was of more historical significance in a way than the left Frazier landed, because of the controversy that followed that KD. Frazier's left hook was, great as it was, merely a confirmation.
I think the single most important punch is the jab, measure your reach with it so you can uppercut and then hook your opponent and knock his butt out! This one works for me.
Roberto Duran's final right on Davey Moore. Signified his reemergence as a major boxing force after the "No Mas" fiasco!
People forget but back in 1894 Fitzsimmons was called upon by the American government because the sun was getting too close to earth. Setting his feet just right he let go a right hook that created a sonic boom to push the sun back and buy us a few billion years extra. That's my pick.