They are not Titles
Titles are likes of "King" or "Fleet Admiral" or "Princess" or "Vice Captain" or "Chief of Staff" or "Shichibukai" ... etc
These are Titles & Ranks & Positions. They are fixed, managed by an Authority & Members can be replaced
But "Strongest Man" & "Strongest Creature" & "Strongest Swordsman" & "Most Wanted Man" & "Marine Hero" & "Pirate King" ... etc
are just Epithets
It's what people call certain Individuals just like other Epithets such as "Heavenly Yaksha" or "The Tyrant" or "Surgeon of Death" ... etc
Characters can have Multiple Epithets, not just one, for example:
WB was called "Strongest Man" + "Man closest to One Piece" + "Whitebeard"
Mihawk was called "Greatest Swordsman" + "Strongest Man" + "Hawk-Eyes"
Kaido was called "Strongest Creature" + "Strongest Pirate" + "of The 100 Beasts"
Ryuma was called "The King" + "Greatest Swordsman" + "Sword God"
Ivankov was called "Okama King" + "Queen" + "Person of Miracles"
... etc