Robinson was always very candid about counting on the expectation that LaMotta would be weight drained for the Valentine's Day Massacre. He stopped Jake with attrition. Fullmer was a different story.
In the earliest telecasts of Robby's middleweight fights, commentators expressly state that he's not a one punch knockout artist at 160. Then, he suddenly puts down an attacking Graziano for the count with a single right hand, and Fullmer with a single hook, both times in a situation where his opponent has all the momentum. Robinson-Fullmer II is the only reason we don't compare Gene's chin with Jake's.
Benvenuti-Rodriguez and Hearns-Duran are obvious choices, but again, Nino did it with a single punch in a situation he was on the brink of losing.
What RJJ did to Hill was stunning, although Virgil did somehow manage to beat the count. (It should properly be recorded as a Referee Stops Contest, or TKO, but boxWRECK still has it listed as a KO. Of course, that's
boxWRECK.)