Best performance in pros he had was vs Briedis, he barely had won this one. In amateur rules rank ofc there some marquee fights: for Oly semi final, final, World championship semi final and final with excellent performance. Usyk was beautiful. How cool he looked in ammy ranks : Euro championship, so interesting was to watch him there.
No it is because speed difference boxers Gassiev and Briedis did had and their skills level was damn different. Gassiev was too inexperienced guy for Usyk and he was slower than Usyk. Briedis too was slower than Usyk but not so much as Gassiev. Ofc Gassiev then was fresh, had a lot of power and excellent stamina, good gas tank. If you consider Gassiev as more experienced operator than Briedis and there: he landed on Dorticos liver blow and managed to use effect from this to continue beat up Dorticos. With punches to head alone he didn't had achieved this. Impressive looking fights: for example : Usyk vs Huck and Usyk vs Glowacki. Or you consider as marquee fight fight vs Whiterspoon? Where he had looked slow and still was capable to finish Whiterspoon? I do agree that vs Gassiev he looked more impressive than in this close fight vs Briedis and there is one big reason: he looked vs Gassiev better because Briedis was high level operator and more versatile, more experienced and faster than Gassiev. As for boxing in depth, best his pro fight was vs Briedis because he achieved his goal there. This was hardest fight he had in pro boxing and Usyk had won this one. Difficulty level too should be considered.
Also because nerds in forums and YT comments value boxers from their origin more than from their actual skills. Because more than 90% internet comments authors about boxing never ever had been punched in the face by someone who really does know what he is doing there. Noobs and nerds in cubicles are internet boxing experts and thinking they are something special and their opinion is important and valuable for real life. It is not real life, this is virtual life for noobs and nerds in cubicles to type comments and enjoy about their skills level and greatness to judge boxers etc. Waste of time if there no covid era here. LOL.