While every emperor is fearsome to some degree, they vary significantly in intelligence. Some are conniving masterminds whose full schemes have yet to be realized, while others are so senseless that they need their crew to make important decisions for them. By ranking them, it becomes easier to categorize their likelihood of fulfilling their plans.
Although Shanks' plans are not yet fully understood, his reputation speaks for itself. So far, he was able to win an audience with the Gorosei and stop the Marineford war all without lifting a finger.
Now that Luffy has defeated two of the biggest power players in the search for the One Piece, Shanks is finally making his move. He knew that if left to their own devices, the emperors would weaken themselves until they were a minimal threat to him. Shanks' opportunistic and low profile methods render him one of the series' most underrated characters.