User blog comment:B214/Question and Answers 6/@comment-106.68.232.55-20161011071720/@comment-10330822-20161021114222

Mid is supposed to end the world for the Norse only, not the entire world. In other words, he kills the Norse gods as part of Ragnorak. Whether that makes him stronger than Fafnir or Tannin is up for debate. Because, by that logic, Tiamat should be sealed away too. Just because Mid is sealed and Tannin/Fafnir are not, doesn't make Mid stronger than them, especially since Tiamat isn't sealed.

The reason the Five Dragon Kings are known as such is that they are all in the same league as one another. Vritra has some OP abilities, so he might actually win in a battle against Yu-Long.

Its pretty much like the Sannin from Naruto. One might win against the other in a straight up fight, but that doesn't mean an enemy the winner can't beat can't be beat by the loser.