Black Blades aren't Pinnacle of Swordsmanship.
Nusjuro doesn't have a Black Blade.
Nusjuro isn't Zoro's Final Opponent.
Post-TS isn't a Different Manga than Pre-TS.
World Government aren't Final Villains, nor are They Strongest Nation/Faction
They couldn't Invade/Stop a Single Island with Pacifista, Seraphim, Super Mega Buster Call, Admiral, Five Elders, CP-0, York
And you tell me They Rule The World?
A Single Giant Group didn't care & you think entire Elbaf Kingdoms gonna be "Oh No, WG gonna Kill us if We misbehave"?
Mihawk was Introduced long before We even learned there is a World Government, and currently They showed like 90% of what They are Capable of while Mihawk barely done anything, so it's obvious who's Reserved to Fall Last
Your real question on who's Final Villain should be "Who's Mihawk's Boss/Inspirational Figure?"
You will say He was Introduced without a Leader, how did you know? Who said his Boss must be a Crew Captain?
The Guy is clearly Religious, who do you think He worships?
Final Villain is simply who Mihawk Worships
You think it's coincidence that O-Tama Controlled Smile Army & Bonney Controlled Pacifista?
You should realize that Poseidon was also the Same, meaning She was able to take Control of Main Villain's Army
So "Sea Kings" are Special Army of Story's Main Villain
If the Army are called "Kings", that means Final Boss is "Sea God"
And He was mentioned multiple times in Story, and it's not Imu who doesn't even come close to Enel's Portrayal of a God, let alone Final Villain
Also Mihawk doesn't Follow Imu, otherwise He would be Pro-WG, not Anti
Final Villain is OPVerse God, Mihawk is his Champion
Blackbeard is Devil of OPVerse, basically Main Secondary Villain
Marines & WG on other hand gonna Fall before these Three