User blog comment:B214/Volume 22 spoilers/@comment-24583889-20160621010300/@comment-24.113.252.213-20160715021730

Oh, yeah.

Great Red, 666, Ophis, Shiva (possibly Brahma and Vishnu) are the only beings we know of that are definitely stronger than Ddraig and Albion.

As for whether or not Sirzechs and Ajuka are also stronger than the Heavenly Dragons, we might find out in the next few chapters, specifically when Issei and Sirzechs fight alongside each other to fight one of 666's bodies (I think that was mentioned in the spoilers).