User blog comment:B214/Question and Answers 6/@comment-190.164.227.239-20161201033222/@comment-9595671-20161205063102

Or perhaps maybe it's how they judge what makes the user "strong", if we considered it just by one's phsyical conditioning or magical skill, basically how fit, strong and muscular they are, then I can see why Issei is the weakest.

He may have gotten more fit when training with Tannin, but it's not obvious either, and Issei is not the best with magic either. I think magic is more likely since Vali's not just a hybrid, but has powerful lineage in his blood, hence why he's stated he'll likely be the greatest White Dragon of all.

So if we considered everything into what defines them as strong, then I think it's easy to say both of them are the strongest Heavenly Dragon users, especially since they've unsealed some of Ddraig and Albion's lost powers.