Here’s the list about the strongest Armament Haki (Busoshoku Haki) Users. Of course the list contains only those characters who have already shown this ability.
10 – RED SCABBARDS
The Nine Red Scabbards are a squad of nine retainers of the Kozuki Family who served Kozuki Oden. They are led by their captain, Kin’emon.
Through teamwork and improved strength and skills of the respective members after 20 years, they were able to fight against the Emperor Kaido and even dealt considerable injury to the latter, a feat that only Oden achieved 20 years prior.
All the 9 Red Scabbards have Ryou, the technique which is hinted by Oda as the only technique which can pierce Kaido’s skin.
9 – MONKEY D. LUFFY
Luffy learned to use Busoshoku Haki during the two-year timeskip and now he is very skilled at using it.
In Wano Luffy has learned how to use advanced Busoshoku Haki, allowing him to project the Haki a short distance without a medium, as well as utilize the higher grade of advanced Busoshoku, allowing him to send his Haki into a targets body and destroy said target from the inside. After training for a week under Hyogoro, Luffy now appears to be able to project his Busoshoku Haki at will, as seen when he was able to blast a hole through a tree by projecting from his fist. Luffy was also seen to be able to remove the explosive handcuffs off of Yamato by destroying them from the inside out, showing conscious control over this higher-grade Busoshoku Haki. Although he successfully dealt damage to Kaido using advanced Busoshoku Haki, Luffy noted that it was only superficial; he later learned to supplement it with advanced Haoshoku Haki infusion to make his attacks tremendously more powerful.
8 – AKAINU / AOKIJI / KIZARU
Akainu, Kizaru and Kuzan have all great mastery over Busoshoku Haki, as shown during the Battle of Marineford, when they used an advanced level of Busoshoku Haki to negate Whitebeard’s attempt to destroy the execution platform, by creating a shield combination of their three Haki.