I consider it the best because of the story:
By your own criteria that you listed, the Leon KO trumps him in every category.
- Max was the underdog and people were predicting that he was doing to get absolutely destroyed like Tony Ferguson. It was a practical death sentence fighting Justin
Leon was a bigger underdog (+235) against Usman than Holloway, who wasn't even that big of an underdog at +135. Yes, people picking Gaethje were predicting him to destroy Holloway but there were a number of sports analysts/Youtubers picking Holloway to win.
- Max dominated the fight and was 4 rounds to zero going into the 5th
Leon was getting dominated in the fight and was on his way to lose a clear decision. Surely that is more impressive than getting a knockout in a fight you were clearly winning and your opponent was already hurt from multiple previous exchanges.
- Max decided to throw down instead of going for an easy decision and got the KO with one second left
Leon was dead tired, looked broken, and only by the words of his coaches and digging deep down was he able to get a KO in the final round.
I just don't know how you top that. Leon being down 4 rounds and getting a KO is up there. Hendo/Bisping is fun. Anderson/Vitor was wild. Barboza/Etim was flashy. None had the story like Max/Justin.
The Leon-Usman fight was for a real belt, with real consequences for the division. Leon knocked out an all-time great, on his sixth title defense, arguably in his prime, and had never been beaten, let alone finished, in the UFC. As a result, Leon is now champion of the world at welterweight.
Holloway won a fake belt in a division that he won't even be fighting in for his next fight, winning a brawl against a brawler who has never been champion and has been finished multiple times in the past. I would say the only thing more impressive about Max's win is that he was moving up a weightclass.
Now, don't get me wrong, Max's KO is on the shortlist for knockout of the year and is one of my favorite of the decade, but surely everything about the Leon narrative tops the narrative in the Max fight.