According to the author of the original novels (Ishibumi-sensei):
In no particular order Ophis, Shiva, Vishnu, Brahma, Śakra, Thor, Typhon (O Fenrir), Hades, Aten and Lugh.
This is how it is. Great Red is not included because it is a creature that basically doesn't fight (I have many claims that this is not true!), And it is the classification before Ophis and Fenrir lost their powers. You should pay attention to the Hindu mythology that has not yet appeared in the series. Well, those who know their mythology know it, but all Hindu gods have similar abilities to cheaters. If I make them appear in DxD, it'll be like the battle in Dragon Ball, so I checked. Hindu mythology will rank high in the top 10 of the strongest that Vali spoke of. It's really scary. If DxD goes on much longer, I'm thinking of doing "Shiva's God's destruction arc" as a last resort. So I don't have any plans to make them appear until then. By the way, the real Sirzechs and the serious Ajuka will be in the ranks.
With all this, visiting and contrasting a lot of information, I took the liberty of creating my personal top of those characters that Ishibumi talks about.
Top 10: Typhon and Fenrir.
Top 9: Lugh the Hero God and Aten Aspect of Ra.
Top 8: Hades.
Top 7: God of Thunder Thor.
Top 6: Crimson Satan Sirzechs and Genius Ajuka.
Top 5: Indra Lord of Heaven.
Top 4: Brahma the Creator.
Top 3: Vishnu the Preserver.
Top 2: Shiva the Destroyer.
Top 1: Dragon God Ophis.
Now, there are other characters that I think could fit on that list perfectly. Ishibumi-san said this list of characters about the dates in Volume 13 of the light novel, so there was still a lot to be done in the series.
Ddraig and Albion would be behind Sirzechs and Ajuka. I have a justification for this. Sirzechs and Ajuka, in their true forms, have the power of 10 Maous, while the Heavenly Dragons have the power of 8 Maous. (Ishibumi Information)
Crom Cruach would be below the Two Heavenly Dragons. I base this on Ddraig showing up to fight Crom in his true form and "tie", but Ddraig does not use his Blazing Inferno of Scorching Flames, so I think Ddraig and Albion could still be stronger than Crom Cruach.
Apophis and Azi Dahaka are said to be at or near the level of the Heavenly Dragons, but it seems to me that they are weaker than they are. This is because Crom Cruach is clearly stronger than them, and he ties with Ddraig without using his special flames.
Lilith would be at the level of Hades, or a little higher. In the novels, it is said that she has 70% of the power of a Heavenly Dragon, which would be the power of 5.6 Maous. Almost nothing.
As for Ophis without the power of Lilith, she still has a power greater than 16 Maous.
It is never said what power the god of the bible has, but personally I think he should have power comparable to Indra's, or even Shiva's. He only dies because he was too weak to seal the Trihexa, who has a power comparable to Ophis and Great Red.
One that will surprise you is Dulio Gesualdo. I think he's stronger than Hades, but I can't tell you if he's stronger or weaker than Lilith. The reason for this is that he can keep a hand in hand with Crom Cruach or Issei in their most powerful version. His Longinus is the second most powerful, and his Balance Breaker is a skill that is broken. To that, we add his power as an angel with 10 and wings and we have a monster.
Sairaorg in his Balance Breaker reaches the power of one Maou, but it's not enough to enter the list. He manages to defeat one of the top 3 of the Rating Games, who is said to have power comparable to a Satan. But if you notice, Sirzechs and Ajuka have a power ten times greater than a Satan.
Cao Cao is listed among the strongest humans, along with Arthur, Tobio, Vasco, and Mitsuya Kanzaki. It is said that Vasco managed to defeat Kokabiel in his youth when he was barely known and not his strongest force, so all the above-mentioned humans I think have the power of a Maou or even a normal god. But not much has been shown of them and I wouldn't know what their maximum potential is.
Cao Cao, in volume 21, manages to do a lot of damage to Apophis with his Truth Idea. But this Balance Breaker is as good as unstable since it is the god of the bible who decides whether it works or not. That is why I refrain from placing it on this list. He is the replacement of Sun Wukong as Indra's vanguard, so I think the level of power of both should be similar.
Tell me if you think one is missing.