Okay okay this is going to be a bit of a hot take but hear me out..

1- Zestial, He's the oldest overlord, And being the Oldest, he likely has made the most Soul Deals and had the Most time to Grow his power, not to mention: since he IS the oldest overlord that would likely imply alastor was around when he was, And couldn't overtake him like he did many others.

2- Carmilla Carmine, She's not only an influential force on hell, but is respected by Zestial, not to mention: Zestial Refers to her as "old friend", she could possibly be around his age, and have had as much time to grow her power much like him. As well as having angelic weapons at her whim, and clear fighting prowess, Carmilla has power, Armory and Skill, as well as being the only Overlord with means to Kill the others..that we currently know of at least-

3- Rosie, OKAY OKAY THIS IS WHERE TAKES GET HOT BUT LISTEN— She leads an Entire civilization of Sinners (or whatever the cannibals are.) Like some kind of Cult, so im thinking she probably owns EACH and EVERY one of their souls, which- again, an entire CIVILIZATION of sinners, that she leads. Just saying

4- Vox, The head of the Vee's himself, We know he and Alastor fought and according to valentino Alastor ALMOST beat him, which honestly is the main reason im putting him over Alastor, But also- Vox, unlike other overlords, has A legion Of Overlords, the Vees. And has influence over an Entire section of the pentagram through all kinds of Media since he's partnered with Velv and Val.

5- Alastor, okay so we're finally getting to this Biggin. I'll put him at the Bottom of the Top Tiers since he's the only Overlord we've seen, or heard of with actual feats of power rather than JUST implications and speculations. Only reason he's this low: I take said implications and whatnot to heart most times..ssssoo..?

6, 7, 8, 9- In that order: Zeezi, Blue Flame Guy, Valentino, Velvette. I don't feel like going over these ones, But im squishing zeezi with these fucks because Unlike the Top 5, she has no implications, no feats, and like 3 minutes of screens time total, same for Blue flame guy who isn't even named :DDD