Kuroobi/Hachi, Bon Clay/Daz Bones were extremely close.
You can shit on Bon Clay, but Oda respected him so much that he dominated and sent Mr.1 flying, and Daz/Bon were totally controversial on some panels in the manga. This was a clear way for Oda to express to fans how strong Bon Clay, was and how comparable they were.
With King/Queen it will be exactly the same, and Mihawk/GB too.
In the other arcs, Sanji didnβt have a serious x1, so itβs unfair to measure it
Well you have made several points but I personally don't agree with your way of argumentation because the statements that you have made to further support your claim are controversial to say the least.
I am glad you kept it short and gave two examples, and lets put the combat power dynamics between Kuroobi and Hachan on back burner for now as that is a more complex yet less important matter. Also if I give my argument for everything it is possible that there is too much to read and we essentially go no where with this debate and waste both of our times. So lets try to establish a consensus between us when it comes to Mr1 and Mr2.
I essentially have very hard time understanding the idea of Mr2 being close in individual combat power to Mr2 because it goes against feats, portrayal and hype.
From feats perspective
-Mr1 was able to match Mr2 in combat, react to all his attacks without even using his devil fruit, for either offense or defense, although with latter its rather tricky as Mr1 does stay in state of being as hard as steel at all times but he did not turn his arm into a blade when he blocked Mr2's kick that sent him flying. But what that really establishes is that at their best when it comes to pure fighting skill, they are equals at best.
-Zoro how ever vastly surpassed Mr1's combat skill to the point where he was constantly overpowering Mr1's stance and Mr1 wasn't able to react to several of Zoro's attacks despite using his devil fruit in close quarters.
-Mr1 was still beating the crap out of Zoro and he said the statements "Blows and blades have no effect on me" which makes sense because Zoro generated Luffy's Bazooka level power with Oni Giri, with which Zoro was able to break steel in prior arcs but he wasn't able to do anything to Mr1.
-So if Zoro wasn't able to damage Mr1 with that level of power and lethality, despite having vastly superior combat skill than Mr1, what chance does Mr2 stand in damaging him? Mr1 seemingly surpasses Mr2 in power, lethality and range but most importantly Mr2 can not do any damage.
-I wouldn't say Mr1 wouldn't have any trouble, like he didn't have with Mr2's goons. He would have some trouble but essentially based on feats he would defeat him with low difficulty effort as he would have defeated Zoro with medium difficulty effort had it not been for Breath of all things. Essentially Mr1 carries the power to kill Mr2 in one attack but Mr2 doesn't carry the power to do any damage at all to Mr1.
From Portrayal Stand Point
-Crocodile made it crystal clear that the lower number means higher combat power and the only exception to this rule is Mr3 because of his special ability. At least as far as Daz Bones is concerned combat power is what he represents above all else, he was known as the assassin.
-Defeating Daz Bones resulted in Zoro garnering bounty of 60 Million where as defeating Bon Clay garnered Sanji no bounty.
-Daz Bones was put a level higher than Bon Clay in Impel down and once again that is also generally based on one's combat power. Like commander level individuals were put in level 5 and level 6.
-The only man Crocodile really seeked after getting released from Prison by Luffy right away was Daz Bones but he did not seem to care about Bon Clay
-There is a well established hierarchy and checks and balances when it comes to Boroque works where if a certain number like Mr8 messes up Mr0 would send Mr7 to kill him. It was even further emphasized when Mr2 was shown in immense fear at the thought of Mr1 coming to kill him. If you want I can find you a panel for that but I believe you are well verse in One Piece to already know about that. Sure Mr2 showed bravado when Mr1 actually showed up but we can't ignore the fact that Crocodile is highly intelligent and he has organized his crew in a way that if Mr2 messes up Mr1 should be effectively able to kill Mr2 without any chance of failure.
So its hard to really judge difficulty when it comes to portrayal, all you can judge is that who is put on a superior pedestal by the author and in universe entities. But once we have established that, we have to use feats to know exactly how big the gap is.
And as I stated above I don't think it would be an effortless fight, Bon Clay is certainly not a fodder and has combat skill to match Daz Bones' combat ability momentarily. But issue is that is where it all stops because beyond that Mr2 can not do any damage, he would have very hard time defending against Mr1's bladed attacks and Mr1 has massive advantage in lethality and power and can kill Bon Clay in one attack.
I would say it would be a
low difficulty fight, perhaps the same as Luffy vs Kaido and it is not a slight to Bon Clay, he is very very strong, is just that Daz Bones is exceptionally strong in every way possible to the point where logically even Zoro must have lost to him.