World's Strongest Swordsman is the only legitimate title which is relevant to one of the main characters of One Piece, Zoro
The other titles have no importance in terms of whether they're just global reputation or actual substantively true
And with Imu's existence as both a man and a creature, WSM and WSC are simply false.
Whitebeard's "World's Strongest Man" is based on his rivalry with Roger. When Roger died, Whitebeard was viewed as the de facto strongest since he was the only one to "truly" rival him. Shiki basically says this, so that's where Whitebeard's title comes from.
Kaido's "Strongest Creature" was purely reputation based on begin with likely because of his insane durability and endurance. People would see Kaido being unharmed by basically anything thrown at him and spread around the rumor that he was the "Strongest Creature".