Nadal playing better in 2013 just means it was a better performance, not a better match. As good as Nadal played, Novak was MIA in 2 out of the 4 sets, which for a player of his caliber, is unacceptable. I don't see how Nadal's level throughout the 2019 US Open has any bearing on which one was a great match. One was a 5 set nail biter with a lot of drama, and one was more or less a dominating performance. It's clear which one is the greater match... Nadal also played much better vs. Federer at the 2008 FO final than he did at the 2008 Wimbledon final. I don't think you'll find anyone arguing the former was a better match.
I don't disagree in the entirety but disagree with several specific points. To me a GREAT match is defined by BOTH players playing well.
Drama is great, but it's not a necessary ingredient. You could have a match in which both players keep choking, the match becomes very close, and ultimately one wins. That match has plenty of drama but the level of tennis is bad. Take for example the UsOpen 2011 Final 3rd set. It was one of the highest levels of tennis you would see, as both players were playing at the top of their game.
The UsOpen 2019 Final was not a great match because neither Nadal nor Medvedev played well. A "normal" Nadal would have won the match in straights. Then the 5th set was also sub-par. Medvedev choked and promptly got broken twice. Nadal choked, got broken once and nearly got broken twice. He even double faulted on break points. His forehand UFEs were throughout the roof. Nadal made more UFEs in one set than he sometimes makes in entire matches. How is this a great match, when none of the players were playing their A game? Since Nadal is a superior player to Medvedev, it logically follows that if Nadal was at his A game, the match would not have been close.
You are comparing the UsOpen 2019 final to UsOpen 2013 - when the latter tennis level was far superior. In the UsOpen 2013 Final - Nadal won 45% of return points - the MOST he's ever had against Novak. When Novak is a far superior server than Medvedev. You say Novak was not playing well, but you forget how good Nadal was during that month in Montreal/Cincy/UsOpen - easily his best hardcourt level ever. 20% of the UsOpen 2013 final had rallies of more than 10 strokes and the longest rally was 54 or 55 shots.. How is that possible if Novak was NOT playing well? He may not be at his peak, but even a B game Novak is better than Medvedev.
Anyway IMO Nadal's 2019 UsOpen Final victory was dramatic, but it's nowhere close to being a great match and not even in the top 5 slam finals that Nadal has played. Rafa didn't play well in UsOpen 2019 and made the matches (QF, SF and Final) closer than they should have been. He barely pulled it out in the end. Most tennis experts (WIlander, Corretja, JMac etc) said that Nadal looked exhausted and his tennis had dipped dramatically. Had Medvedev been more experienced he would have likely won given Nadal's level in the Final.