usually Oda put some kind of way to tell us who's stronger than who for example
Pirates = Bounty
Shichibukai = former bounty
Marines = Rank
Baroque Works = Number
CP9 = dourikiGod and Priests = survival percentage
but i want to talk about two power Rankings not yet discussed and those are
1: The order of weakest to strongest people in thriller bark
my opinion: Luffy, Gecko Moria, OZ, Nightmare Luffy, Shadow Asgard Moria, Kuma (you can go further back if you want i don't care)
2: Luffy's forms from weakest to strongest
chibi Luffy, Luffy, gear 2nd, gear 3rd, Nightmare Luffy, gear 2nd + gear 3rd, and afro Luffy(just kidding… but I do get a feeling luffy may at some point unlock some power related to the Will of D.)
3: and even though I wasn't originally planing on it, the straw hats from weakest to strongest
my opinion: usopp, nami, chopper, brook, franky, robin, sanji, zoro, luffy
(the ones I'm not sure about are usopp through brook, and if Oda didn't make sanji always doing just as good as zoro at some things [mostly like when they faced marines at enies lobby, or when facing buggy crew, or BW members] or the whole monster 3 thing on thriller bark I'd say robin would be better… and that he faced the 3rd best out of BW and CP9)
go ahead and comment or list your own ratings on these matters