WSS Title is a Mystery just like One Piece, All Blue, Void Century, World Map ... etc
We know the definition of World Map, All Blue, All Healing Cure ... etc but the Mystery is how are they created and found, the same applies to WSS, we know it's Mihawk, but how? That's a Mystery
However by simply checking Zoro's Journey, we can discover what WSS means
Against Hatchan, we learned Sword Quality matter
Against Mr. 1, we learned of BOAT
Against Ohm, we learned of Flying Slashes
Against Kaku, we learned of Asura
Against Hyouzou, we saw CoO
Against Pica, we saw CoA
Against King, we saw CoC
TBC
In other words, for Mihawk to be WSS that means:
He has Greatest Sword/Blade
(Yoru is not only Supreme Grade but also Black, and Ultimate One-Sword Style Weapon)
He cut can anything
(His BOAT is Mastered enough to cut all Materials and Elements)
He has Strongest Flying Slashes
(This requires Physical Strength on highest Level, Mihawk's Shockwaves alone from swinging can cut Mountains from far far away)
He has Mysterious Will
(This one isn't explored yet, but it's related to his Double-Layered Eyes, just like Hakuba & Imu)
He Perfected CoO
(He isn't named Hawk-Eyes for nothing, previously was gonna be Clairvoyant, his CoO is highlighted and Databooks claimed he us extremely talented in reading others and their Power, hence why Oda chose him to describe Luffy's Best Ability)
He Perfected CoA
(Its clear that if he isnt lacking in this, cuz he can't beat other Swordsmen even if they lose their Weapon as Pica explained, and as hinted by his Black Blade and Smoke similar to KOH in certain Non-Canon Art, he seems to have his own Perfect Non-Temporary KOH)
He Perfected CoC
(This one isn't shown yet but strongly hinted and it's same case as CoA, and seeing how Zoro's Presence is really hyped and how Mihawk made Luffy visualize his Defeat and how he rivaled Shanks, it's clear his CoC is Highest Tier)
In conclusion:
Mihawk is WSS because he has Ultimate Skills combined with Ultimate Sword, Ultimate Haki Mastery and Mysterious Willpower which are all strengthened by Top Physical Stats and Mentality.
Oda even went further and gave him Religious Theme and implied he is seen as Handsome as Hancock and is Skilled in everything such as Navigation, Farming, Cooking and even Sniping
To put it simply, Oda wants to portray Mihawk as "The Perfect Man/Fighter", who will be Defeated by Zoro who is Stupid most times outside Combat, only Sleeps when not Training, received many scars, tasted defeat 2000 times, lost his Swords few times, can't find his way or directions ... etc
The idea is that Hardworking, Inherited Will and fighting for a greater cause can surpass the Perfect Skills
