User blog comment:ImperiexSeed/The oldest being, archangel or leviathan?/@comment-216.10.220.114-20120514050902/@comment-4920092-20120514143932

Doubtful. And there's no way you can possibly be 100% sure, since none of the creators have spoken about it.

And I think there are other beings that take precedence over the Leviathans, such as Archangels (which is more or less confirmed, since it didn't work on Lucifer, and it's pretty much unanimously agreed that the 5 things are collective races) and the Horsemen.