Arguello I is the natural selection. In round 14 of a debilitating war, he staggered Alexis, and then unloaded a relentless barrage until referee Christodoulou made one of the most perfectly timed stoppages in championship history. To produce an explosion like that, so late in such an epic war, is a mindblowing display of sustained aggression. Most such late stoppages are either the result of the vanquished steadily wearing down, or largely from the administration of a single punch. What Pryor did (coming after a round in which Arguello scored one of the most perfect right hands to ever fail to wobble the recipient), was an extremely rare fusillade of withering championship round offense. In HW competition, Marciano couldn't do it in round 15 of his first match with Charles, and Ali was actually fighting back after getting up in the FOTC. Typically, the sufficient reserve firepower needed to sustain and induce such a late halt to the action has been exhausted. Pryor never had Arguello so seriously stunned in their rematch, as Alex seemed to go down to get breathers (like Archie Moore occasionally would). Arguello made a volitional choice to finally accept the ten count. But if Pryor stunned somebody involuntarily, their only hope would be that the round was about to end.
I considered Zarate, FG, but remembered that he let Pintor get off the deck to go the distance when Lupe took his title (an extremely bad moment in which to finally fail to stop a distressed challenger).
Gomez was another who I considered. Nobody's perfect, and the thread asked to name only one. I remembered that Wilfredo had Sanchez stunned, but couldn't close the show. There were also times when he chose to go easy on a hapless victim (former Nigerian amateur star and peek-a-boo stylist Eddie Ndukwu), and he needed the referee's intervention to finally stop former amateur conqueror Derrick Holmes (who had the much slower Gomez out on his feet in the first couple of rounds), despite dropping Holmes seven times in two rounds, and fracturing his jaw. (Incredibly, the referee who let it continue that long was Joey Curtis.) But Holmes was never close to taking a full count, and would have continued getting up for as long as it was allowed to continue. (This was on NBC's Friday Night Fights.)