The Secret of the Gorosei tells us who is the Strongest Elder and who is the Weakest Elder.
The Ope Ope no Mi is a Devil Fruit in the One Piece world that grants its user the ability to perform the “Perennial Youth Operation”. This operation allows the user to exchange their life for the ability to grant another person eternal youth.
Many theories suggest that the Gorosei are not ordinary humans but individuals who have been alive for a very long time, possibly centuries. To do this they would have received the the Ope Ope no Mi Surgery Operation. This would explain their influence and authority within the World Government. They may have manipulated the Void Century and the history of the world to erase any mention of their involvement with the Ope Ope no Mi.
With their long lifespans, the Gorosei could have continued to exert control over the World Government and the Marines.
Let‘s say the “Perennial Youth Operation” will not make the targeted individual “young” again but just won’t let him getting older. Then this could be the Gorosei ranking order.
The youngest would be the most important (and strongest) Elder since he would have been the first to receive the “Operation Perennial Youth” after Imu.
In fact we know so far, that when a devil fruit user dies (and that would happen with the Operation Perennial Youth), the fruit will reappear in a random other fruit. And to find that devil fruit 5 times over and over again, time passes. Soooo by that logic, it would make sense they used it at the most important Gorosei first and the weakest one in the end.
I don’t know if Saturn or Valkyrie is older but the chance that Saturn is the weakest is high (also supported by shonen rule – it would make sense for our protagonist Luffy to face the weakest of them first).
*Theory by crack4pot
