Great question, as always bro. I really enjoyed the the answers, it will be almost impossible for me to not duplicate many of the fights already mentioned but I will do my best.
I will take "punches" in the literal sense, and the most famous one had it's own name. Rocky Marciano's "Suzy Q", kept Rock unbeaten through 49 fights and was responsible for 43 KO's. Few punches in boxing history actually had names, so Rock's right cross lands at number 1.
Joe Louis's right cross has a huge forty foot duplicate of itself outside the arena that is named after him in Detroit. I am certain no other punch had a monument built to it of this stature, it's truly an impressive piece of art, much like the punch that KOed 50 plus heavyweight's during Joe's historic career. When Max Schmeling went down, Joe's right cross turned off all the Radio's in Nazi Germany, believe me it earned the monument and the arena name.
Sugar Ray Robinson's left hook did more than take Fulmers title and make another Rocky stamp his leg to try and get the feeling back, it put close to half of Ray's hundred plus KO victims to sleep, so the Robinson left hook edges Joe Fraziers left hook to get the 3nd spot,
Thomas Hearns had a right hand like a cannon and it not only produced the Duran and Cuevas classics, it made Tommy a 9 time world champion and the first 7 division champ in boxing history. The Hit man's vaunted Right cross lands in 4th place on my list of deadliest punches. It stopped 48 of Tommy's opponents inside the distance behind that "Cobra" like jab.
George Forman's right hand produced the highest KO percentage of any heavyweight champion, not to mention made Big George the oldest heavyweight champ in boxing history. Over 21 years after his destruction of the great Joe Frazier, George's right hand put away Michael Moorer to land Foreman's right cross at #5 on my list.
The Larry Holmes left Jab was about as deadly a punch as any bomb dropped by history's heavy hitters, and probably the best jab in the history of the Heavyweight division. It almost got Larry's record equal to Marciano, and it landed with such accuracy the Holmes jab placed 6th on my list of history's deadliest punches.
Joe Frazier's left hook lands as the 7th deadliest punch in boxing history, but arguably could be ranked higher. Joe, like Marciano was never a defensive specialist, and the speed and power of Joe's left hook made it quite possibly the best left hook in heavyweight history, not to mention made Joe one of histories most respected heavyweight champions.
The last three spots could arguably be filled by at least a couple dozen of boxing's great punchers, so I will go with my personal favorites and let the chips fall where they may. I know it will be impossible to get even a consensus on these three, even I can come up with many alternatives, but from a personal standpoint they stand out as significant.
Michael Carbajal's left hook did more than salvage the fist Gonzales fight, it made Michael the first little guy to clear a million dollar pay day in boxing history, so I put Carbajal at number 8 knowing full well a lot of others will disagree.
Gerald McClellan's tragic clash of heads with Nigel Benn does not change the fact that Gerald destroyed the hard hitting Julian Jackson twice and was in the process of dismantling Benn with hammering right hands before the injury. I put Gerald's right cross at number 9 well ahead most of history's 154 lb sluggers, Gerald was a harder puncher than Jonh Mugambi or any other of his contempories. 29 of Gerald's 31 wins were by KO and there would have been many more had tragedy not shorted his career.
The last, and certainly not a unanimous choice for this star studded list is the straight left of Manny Pacquiao. If we just list his recent accomplishments against great Mexican champions of recent note it puts Manny in exclusive company, and he gets the 10th and last spot.