The criteria is probably their rating. Since they use the ELO system, each game gives or loses points. So the ratings are a good measure in how good you are over time and meeting multiple opponents.
And Carlsen absolutely dominates the others, and have been the top rated player continuously for a decade. So no questions about who is the best, WC title or not.
Carlsen is right now 98 points above Nepo, the challenger, which is an insane difference.