By skill, Floyd Mayweather. By achievements, Manny Pacquiao. That being said, I believe Terrence Crawford would’ve beaten both of them at 147 pounds.
Lomachenko. On pure skill. Turned pro too late though , made poor career choices & was stuffed by the the establishment. Not talking resume, talking pure ring genius.
Mayweather, hands down. This is easy. Next owner of it will probably be Inoue. Especially if he beats Nakatani and Bam. Crawford had the potential but didn’t fight enough. now, if Bam goes on some epic run and takes out both Inoue’s and Nakatani, it could be him. Not the most likely outcome but who knows. He’s a unique fighter and every time we think someone is going to give him issues, he knocks them out. So… ? 20% chance here. If Vergil would ****ing fight, he could get into this convo. His combo of skills, aggressiveness, and power is something mean AF at his weight.
Naoya Inoue, Usyk, Mayweather, Manny Pacquiao, Junto Nakatani, Nonito Donaire, Pongsaklek Wonjongkam (for sure the best Flyweight in the 2000's for me with Nonito Donaire), Mairis Briedis, the fourth kings (Estrada, Chocolatito, Srisaket Sor Rungvisai, Carlos Cuadras) marked the history of the Super Flyweight, Kazuto Ioka also (the Japanese veteran beat Fernando Daniel Martinez in the second fight for me, He have a exceptional longevity and has beaten 2 champions in four weight classes : Donnie Nietes and Kosei Tanaka + a solid theee weight classes champion with a victory against Akira Yaegashi, perhaps not at the level of the others mentioned apart from Cuadras and Srisaket Sor Rungvisai who had smaller resume than Ioka, but he deserves to be mentioned, he established himself as one of the best 105, 108, 112, 115 lbs of all time and to be honest probably the most dangerous Minimumweight since Ricardo Lopez).
And if Kazuto Ioka beats Takuma Inoue he will deserve his place here even more, Takuma Inoue is still the number 2 at the moment in Bantamweight.
He lost to Lopez when he was in his prime, and Haney when he was a bit older, couldn’t deal with Salido even if he was dirty. Lomachenko may have been an amateur great but he’s not near Golovkin, Usyk, Mayweather, Pac, Inoue etc.
I'm not sure that Usyk isn't the most skilled fighter of the 21st century. He is definitely more offensively creative than Mayweather was.