User blog comment:B214/Question and Answers 6/@comment-185.112.105.215-20161212173441/@comment-29963364-20161212190847

From what we've seen/heard/read in the story so far...

Tannin = Ultimate/Satan-Class Devil (Since he's no longer a one of the Dragon Kings, you don't need to count him)

Midgardsormr = Ultimate-Class Devil (Assumption)

Yu-Long = Ultimate-Class Devil (Assumption, since he has experience fighting alongside Sun Wokong in the past)

Vritra = Ultimate-Class Devil (Assumption, since he has potentially OP, Cheat-like abilities)

Tiamat = Satan-Class Devil

Fafnir = Ultimate/Satan-Class Devil (Super Devil-Class when in Outrage Mode)

From this, we can assume the Dragon Kings are each Ultimate-Class at the very least. If Fafnir can fight and corner a Super-Class Devil in Outrage Mode (although Rizevim was weaker than Sirzechs and Ajuka), then it can be assumed that if all the Dragon Kings entered Outrage Mode, then they would easily surpass the Four Great Satans in power combined (without Sirzechs and Ajuka going all-out).