At this point, it is a given that
Advanced Conqueror > Devil Fruit Awakening/other special abilities > Haki > Devil Fruits/other abilities. Between characters of similar levels, the ones with stronger Haki are, most of times, the stronger ones. This really seems the way the author sees things in his own series.
Kaido already knew well about DF Awakening, and then witnessed the Awakening of one of the most powerful DFs, possibly even the most powerful one, the Gomu Gomu... ahem the Nika Nika.
And yet he still says that Haki surpasses all the powers.
So yeah, the strongest characters in One Piece are the one with the strongest Haki.
In a fight between two characters of similar levels, the one with the strongest Haki will be the winner.
This is so coherent, after all. Will, being it inherited will or willpower, is one of the main themes of the One Piece series.
Haki is the power of the will.
Willpower makes you tougher, that's
Armament Haki. Willpower makes you feel the surroundings, that's
Observation Haki. The strongest kind of willpower is the one that makes you dominate over anything and anyone else; and that's
Conqueror Haki, the act of the Kings imposing their superiority to the lesser ones.
That's why Haki the strongest power in the world of One Piece. As Kaido said, in a direct comparison with the other abilities:
"HAKI SURPASSES IT ALL".
And the strongest kind of Haki is the Advanced Conqueror Haki, which indeed was portrayed as the power of the Top Tiers, through Kaido's words again
"Infusing things with Conqueror Haki? ONLY A HANDFUL OF THE VERY STRONGEST CAN!"
Therefore, you could say that, being Adv CoC the strongest version of Haki which is the strongest power, the actual strongest power in the verse is Adv CoC. So basically it's like this:
Adv CoC Haki > Haki > anything else.
Doflamingo, being a Conqueror Haki user and having a DF with Awakening, and also perfectly knowing what Ope Ope (one of the strongest DF around) can do, so having massive knowledge, refers to
being a Conqueror as a proof of the greater power of the ones who possess that ability compared to the ones who do not, as
something that creates a clear distinction between who has it and who doesn't have.
Kaido's statement then creates an even further distinction between who has it, and who has it in the Advanced version. Because yes, even among the already very few ones who have Conqueror Haki, only the present strongests or the ones with the greatest future potential are able to use Advanced Conqueror Haki. Confirming that (Adv CoC) as the strongest version of the strongest power (Haki), and making it an established landmark in the power levels of the series, with hype and portrayal above 9000.
Hundreds and hundreds of chapters ago, the idea that the one with the stronger Haki is the one that most of times is the stronger one overall and the winner of a fight, is textually suggested in the manga.
A character textually states that defeating his DF means nothing because he will still win thanks to his high level of Haki; and the answer is that's true, it will depend on who truly has the better Haki for real;
the winner will be the one with the strongest Haki.
Now that may not be true for 100 comparisons out of 100; but for the most part, yes. Especially characters with Advanced Conqueror, they are among the strongests for the mere virtue of unlocking and wielding that power. There's a reason for why people like Doflamingo, Chinjao, Kidd or Katakuri are Conquerors and very strong guys, but still could not manage to obtain the usage of Advanced Conqueror Haki.
After all, all of this should not come as a surprise.
Not only it is perfectly coherent with will being one of the main themes of the series, and Haki being a reflection of the character's willpower.
Also, please notice that every past, present and future Top Tiers have all different fighting styles: some are DF users, some are swordsmen, some are brawlers and so on. But all of them have exceptionally strong Haki. All of them.
There are so many Top Tiers who do not have a Devil Fruit: Roger, Rayleigh, Mihawk, Shanks, in the future there will be Zoro who is already rising at the highest levels; Oden too can be seen as at that level. Garp as well
.. and he didn't even use swords or other weapons, only his bare hands and Haki!
While, on the other side, there is no Top Tier that doesn't have exceptionally strong Haki, and this says all. Even the Admirals use their DFs a lot, but still they are Haki masters, just see how easily they blocked Whitebeard's Gura Gura with their Advanced CoA.
So, yeah,
Haki is the strongest power in One Piece: as Kaido said
"Abilities are not enough to conquer the world... Roger didn't have that... because only Haki trascends it all!"
And
the strongest version of the strongest power is what Kaido called the power of the Top Tiers, the power that
"only a handful of the very strongest can", aka the
Advanced Conqueror Haki.
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