The simple non-troll answer to your question is there was no clear cut strongest in the eyes of the world in Roger's era
Xebec Roger Rayleigh it could be either
It is brutally honest that Mihawk is strongest and gap between Mihawk and second strongest shanks is so big Shanks isnt even worth his attention anymore.
Oda literally spelled it out for us that exact same fact at least 10 times by now. In Manga, during Mihawks bounty and data books
@Shiroyru @GalaxyLevelShanks
WSS isnt some Island you are chasing by doing 100 Tricks, stealing properties, and begging your rival to rent him his minion for a few months.
You simply have to be so strong, world wide consensus is that you are the strongest.