I would really like to join this thread, because I acually like discussing "powerlevels", but I feel like I need to explain my points of view first because I am also interested in how you evaluate powerlevels in general.
1. There is no Character A > Character B > Character C kind of powerlevel chart. The vast amount of different special abilities makes this impossible. There are only rough brackets were characters can fit in.
2. The main characters don't jump strenght tiers bewteen arcs unless it is expicitly stated. The Luffy that defeated Alvida is the same Luffy that defeated Crocodile and Enel and the same Luffy that lost to Kuzan. Afterwards he developes the Gear Techniques, which is a straight powerup. Before, what he was gaining was only expierence, you can see most of the Straw Hats trying out weird and ineffective techniques during their early fights (Gomu Gomu Net, Nami's first tact, Usopps early arsenal), while becoming more min-max oriented, flexible and adaptive later on. Keep in mind that the Straw Hats journey takes place in only a short amount of time. Given that the average age of the supposedly strongest characters is a lot higher than the Straw Hats, it is impossible for them to jump tiers every arc. Jumping Tiers by more than one was what the timeskip was all about. Now progression rate has slowed again. Luffy that defeated Hody is the same Luffy that defeated Cracker. Only after the Katakuri fight he gained a direct powerup, advanced CoO. Now things will slow down again until the next and probably final (as we are at 80%) combat related powerup, most likely "Awakening".
3. As I said above, Experience is a much bigger factor than strenght. Luffy falls for every trick his opponent has upon first encounter, best recent example is Luffy immediately getting stuck in Katakuri's mochi upon their very first contact at the wedding, but the best overall example is probably Luffy vs Foxy. Experience is also the reason why Crocodile could clash evenly with Mihawk during the Summit War, while Luffy realized he couldn't. Luffy had beaten Crocodile before, so why does he perform so much better fighting wise during the war? It is because he is a seasoned pirate that knows what he is doing, while Luffy was a rookie.
4. What characters say about other characters has to be taken with a grain of salt. The discussion above about a supposed "Whitebeard tier", I cannot understand it. Whitebeards feats are undeniably insane, he has the Gura Gura no Mi and can keep fighting for a few minutes with half his head gone. But he also had half his head taken off by a marine admiral in close quarter combat, and he himself ultimately did not manage to take out this marine admiral or any admiral or the hated Teach for the rest of the war, and on top of that had heart strokes in the middle of combat. Doesn't really look like the "strongest man in the world" to me. More like he was once, due to his rivalry with Roger, and that's where his legendary status comes from, because that is how legendary statuses work. They come from reputation, not from actual feats. Marineford Whitebeard would be in the same rough bracket as the other Yonko and Admirals. There really is not a single feat during the war that sets him above the rest except his durability but he is also the only one to die.
Another example is Luffy as the fifth emperor now, but here it is much easier to catch that it is just Morgans giving Luffy a reputation boost and not actually true.
5. Titles and bounties are related to strenght but not directly. That means they can't be used for direct comparison. We can't say "Yonko is stronger than (Fleet) Admiral", and we can't say "Yonko Commander is stronger than Vice Admiral". All are just titles within their own institution (Pirates/Marines), and do have a relation to strenght but not a definite one. And "Yonko Commander" technically doesn't even exist as every crew has their own ranking system.
Same for Bounty, we can't say X is stronger than Y because his bounty is higher.
All that said, I think it is extremely difficult to figure out a good chart that places all characters in the right spot. What I find uninteresting is that most of the powerlevel discussion resolves around the top, which for me, is the easiest to categorize. Honestly, I've been reading about Whitebeard vs Akainu on every place in the internet for 8 years now, it's getting tiresome.
I would love to talk about some more mid-tierish brackets rather than about top tier specifics like "Who is the strongest Yonko?", or is "Doflamingo stronger than Cracker?", because in these cases we can't give definite answers without driving into fanfiction because the characters are so close in power anyways.
To also start a new discussion, I would like to to ask if you think these characters are comparable to each other, thus forming a rough bracket:
Perospero, Daifuku, Oven, Vergo,
Pedro, Tamago, Pekoms , Sicilian, Sanji
Also, I would like to ask where would you put Pica, Bege and Smoker (or any other comparable character) in this list.
