The one who is all odds stack against him but still managed to win through ferocity will and determination.
Duran did that against Barkley using both great defense and offensive skills plus guts a great chin and a three punch combo that knocked Barley on his ass.
The most impressive fight is a subset of the first. The best fights in boxing is when the outclassed, outboxed, outskilled and outgunned fighter comes back with sheer heart, skill, chin and knockouts the opponent from behind the scorecards. AND we saw an example of that tonight!!!! The most impressive fight of the night was Ceja vs Ruiz. Ceja was getting whooped on the outside, was less skilled and out smarted. But the smaller fighter pulled up from a KO, survived...went back out there, timed a perfect shot and finished his opponent with everything he had! THOSE are the most impressive fights period. Its not a technical masterpiece, its not a slugfest, its when the lesser fighters goes against logic and defeats the technically better fighter.
To me when a fighter is at a disadvantage(s) and he can do either one vs a great bigger man, that is impressive. BUT... If I had to choose with a gun to my head, ko is definitely more impressive. Because the winner is at a risk of getting ko'd himself, but he's going in for the kill. Like a boss.