Maybe Foreman or Shavers qualifies but the best I ever saw was Julian Jackson. They way he flattens Negron, Norris, Cardamone and Graham is remarkable. Too bad his defense was not better.
Why Foreman? He only knocked out one man using a single punch in his entire career. He was an extreme hard puncher, no doubt, but there are harder single punch-punchers.
I agree for the most part. A lot of that has to do with style too. Every punch from Foreman hurt though. He'd stun guys with his jab all the time. Tyson got most of his KO's from barrages. It's usually guys who are freakish for their weight who have the memorable one-punch KO's. Jackson and Hearns came to mind for me as well.
I think Tommy gets more credit for perceived potential than actual achievement. Cuevas beat the count by a comfortable margin, and Hearns had previously stunned and dropped Duran, prior to that final body stiffening right. Shavers probably is the hardest puncher in boxing history, but most of his opponents were prepared for his power, and his accuracy left something to be desired. Aside from Ellis, I can't think of a situation off-hand where Earnie dropped an opponent for a count of at least ten with a single punch. Arguello could do it with perfect placement, rather than sheer force of power. His left hook in the Escalera rematch and measured right cross against Rooney are good examples of this. One punch knockout power in both hands is exemplified by SRR/Graziano (with Ray's right) and SRR/Fullmer II (with Robby's "perfect" hook). From the clips I've seen of Julian Jackson, he might be an excellent candidate. Marciano and Cuevas are supposed to have done this frequently when moving up through the ranks. Weaver did it with the hook against Tate, then the right against Coetzee.
From the few fights I've seen of his, Jackson. You feel like calling 'Timber' when Herol Graham falls. Not too far behind him I'd include Nigel Benn and Gerald McClellan, was watching some of Benns early fights today, I was ducking in my chair. Certainly not a purists delight but undoubted power.
Mike Tyson, Rocky Marciano and Lennox Lewis have more one-punch KO's AND faced much better opposition.