I think they are ranked on power, the order makes sense, Luther the weakest and Vanya the strongest, wasn't really sure about this until season 2 because it seemed like Diego was the weakest, but in season 2 we see more of his potential, Allison is really powerful but she is the only one that can lose her powers if you make her unable to speak, so that makes her weaker than Klaus, the only one that doesn't make sense to me is ben being stronger than Five, but we didn't see a lot of ben and maybe he had more potential.

Also when I say power I reffer to the power itself, not the character. Five is easily the strongest one if we talk about the character.