In terms of power i rank them according to:
1 - Feats and versatility.
2 - Hype and portrayal.
As a source i use the manga and the manga only.
The databooks are full of hyperboles and a characters' stats means very little, its not the best way to judge their skill or importance.
And i also place characters above others even if they lose against them. That's where versatility comes into account but happens mostly after the top 15 where the power levels aren't that high and overall skill matters more. Its impossible to make a 100% accurate list regardeless, since there will always be bad match-ups for certain characters.