There needs to be a certain amount of info to do lists.
I can do some stuff like say X is supernova level, Y is Shichibukai level, Z is supernova level.
One thing that makes it easier for Naruto is jutsu have certain levels, and ranks are more closely related to level. Like, you know if someone has sage mode, they're are least N strong. And a jonin *is* going to be at least N powerful at minimum, ditto chunins, Kage, and so on, so that helps too. Plus, we've got databook stats.
While with OP, you mostly have to rely on seeing their complete fights, and then if it's a fight where you get to see them pressed hard, since unless someone's pressed it's hard to get a good picture of their power.
Ranks, well, the guy who sold out Nami to Arlong is a captain, ditto Axe Hand, and so was Smoker to begin with, and Captain Hina. Nami with her weather staff could without a doubt easily handle rat captain and axe hand at the same time no problem, but Captain Hina casually beat a guy who gave Sanji a very tough fight.
Location affects power too- a Grand Line LT is likely stronger than most East Blue captains.
It's not until the Admiral ranks (including vice and rear) that a certain level of power seems required.
Sure, why not? We'll start at the top and work our way down.
Alright, as it breaks down, I'd say the three major high-end categories are Yonkou/Admiral, Shichibukai/Vice Admiral, and Supernova (which seems to correspond to rear admirals and commodores).
We can split each into three subsections, high, middle, low. I'll start with 2-4 of each.
As per usual, within each level I'll do alphabetical
High Yonkou-
Gol D. Roger
Whitebeard
Mid Yonkou-
Aokiji
Akainu
Kizaru
Silvers Rayleigh
Low Yonkou-
Portgas D. Ace,
Blackbeard (shadow fruit)
High Shichibukai-
Kuma
Trafalgar Law (post skip)
Mid Shichibukai-
Vergo
Jimbei
Boa Hancock
Low Shichibukai-
Caesar Clown
Crocodile
Gekko Moria
Smoker (Vice Admiral)
High Supernova-
Eustass Kid (pre-skip)
Luffy (with gears)
Monet
Rob Lucci
Trafalgar Law (pre-skip)
Croc did lose to Luffy (with a little weakness exploitation), Ace has stronger attacks, has Haki (he would without a doubt beat Croc in a fight for this reason), could fight even when his powers were being cancelled, and was being groomed to take Whitebeard's place. I could see knocking him down to high Shichibukai. Think I should do so?
Apoo? Higher bounty, better powers. I could see putting him down to the same level, though.
Though we haven't seen a lot of him, he is the third highest bounty*, and his abilities are fairly hard to deal with. Mid or high, not sure which. I'm temped to go mid, since he didn't show too much in offense powers.
(*Bounty's not everything, we know Law's is lower, but he also likes to keep his head down)
Yea, he's really strong, but on the flip side it's purely physical strength.
Low Yonkou is a safe bet, in terms of sheer power and control over his Fruit, he'd rank even higher, but he is not nearly as impressive physically as his peers in that rank.