Current Ones:
Based on Royalty/Position:
Hancock - Pirate Empress
Pizarro - Corrupt King
Riku - Miracle King
Elizabello - Fighting King
Inu/Neko - Day/Night Rulers
Alber - King
Queen - Queen
Imu - King/Queen of The World
Based on Achievement:
Brook - Soul King
Roger - Pirate King
Rayleigh - Dark King
Yonko - Emperors
Sabo - Flame Emperor
Orlumbus - Massacre Ruler
Based on Pure Power or Ability:
Zoro - King of Hell
Usopp - King of Snipers
Crocodile - Desert King
Ivankov - Okama King
Kaido - King of Beasts - Dragon King
Ryuma - The King
Other Epithets that indicate Supremacy:
Usopp/Enel - God
Mihawk - Strongest/Greatest Swordsman
WB - Strongest Man
Kaidou - Strongest Creature
Buggy - The Star Clown/Jester
Big Mom - Strongest Woman
Kid - Captain
Cavendish - Pirate Prince
Doflamingo - Heavenly Yaksha
Shiki - Golden Lion
Sengoku - The Buddha
Garp/Koby - The Hero
Dragon - World's Worst Criminal
Nola - Master/God of The Sky
Oars - The Devil
Neptune - Sea God
Ryuma - Sword God
Du Fled - God of Fortune
Shirahoshi - Poseidon
Doc Q - Death God