Strongest non devil fruit User
Red Haired Shanks
While his powers haven’t been shown on screen he is undoubtedly the strongest non devil fruit user in One Piece. Mihawk mentioned that they used to fight a lot but since Shanks lost his arm Mihawk lost interest. He was able to get past Kaido’s raid on his fleet which is something that not a lot of people can say. He wasn’t afraid to clash with Whitebeard, who is so powerful that he makes the Admirals shake in their boots.
Silvers Rayleigh
He is the former first mate of the pirate king and a powerful swordsman and master of 3 forms of Haki. While he has long retired from piracy and is in his late 70’s the Marines still want him dead because he still can square off against admirals and cause crippling damage, and the fact that he just sailed right into Marine HQ without breaking a sweat is a testament to that power.
Dracule “Hawkeyes” Mihawk
The world’s greatest/strongest swordsman, he only fights with his swords and haki. So far no one has been able to take his title from him and a few dare try to challenge him. He is strong enough that even Doflamingo talks to him with respect and the Marines hold him in high regard.
Monkey D. Garp
A powerful melee fighter and user of Haki. He has lived through several generations of great pirates and come out as a top marine who wasn’t beaten and toyed with. He cornered Gol D. Roger several times, subjugated Don Chinjao and fought off Blackbeard’s core crew during the great war. He was strong enough to become admiral but passed off the position.
Update: I just realized that I forgot another major character who is crazy strong.
Knight of the Sea Jimbei
He is the most powerful among his own species and has garnered a lot of respect and attention among pirates for his tremendous strength. He has clashed evenly with Ace during their first encounter for five days and nights, easily defeated Arlong who gave Luffy a hard time. He is also fearless which makes Big Mom unable to extract his soul and doesn’t hesitate to go up against Big Mom and try to stop her. His strength can easily change the tide of battle and his haki skills are nothing to laugh at.
Gold D Rogers ( NO .1 )
Unknown so far.
Shanks is an Emperor, but implied to be one of the weakest, mainly because he's the youngest.
Commander Thatch, from the Whitebeard Pirates, explained to Ace that for him to become an Emperor he has to at least become as strong as Shanks, and therefore, seemed to imply that Shanks was the weakest of the Emperors.
Shanks doesn't have a Devil Fruit either, unlike the others Emperors, but seems to focus mainly on his swordsmanship and his Haki mastery.
Shanks also has an extremely powerful crew, and great influence in the world.
Mihawk is a strongest swordsman, he’s the "World's Strongest Swordsman", and with the little info we have so far, seems to imply that Mihawk is stronger, or at least, relative to Shanks.
Mihawk on his Vivre Card is stated that :
"Not only can he discern a person's talent with a sword, he can also see the true strength of those that would stand before him".
This was shown in the manga itself when he sees Zoro and mentionned that he can discern the strength of his foes with just a glance.
Yet Mihawk no longer sees Shanks as a worthy challenger and now waits for someone stronger than him to appear.
According to the various Databooks and Vivre Cards :
"Shanks was once as powerful as the world's strongest swordsman (Mihawk)".
Seems like losing his arm really hampered his abilities.
Many also like to downgrade Mihawk because he viewed himself as inferior to Whitebeard during Marineford, while Shanks was Whitebeard’s "equal".
First of all, while Shanks clashed with Whitebeard, that doesn't mean they were equal. They were both Emperors, yes, but with different strengths and weaknesses.
Secondly, Mihawk never clashed with Whitebeard before, and furthermore at this point, Whitebeard was still considered as the "World's Strongest Man" and the strongest pirate in the world, which in the eyes of the world, and of Mihawk, rank the old man above mankind in general, including Shanks.
Also, the true translation of Mihawk who wanted to test his might against Whitebeard was that, Mihawk's raw japanese text :
"The real/true distance between that man and us looks close/rather small".
Or something like : “Perhaps an idea, yet it seems the distance between that man and I may be small.”
What we do know for sure is that currently, Mihawk is unrivaled as a swordsman, and is the strongest.
“He sits atop as the strongest, gazing down at his prey. The World's Strongest Swordsman!
He rose to renown even before the Great Age of Piracy. Even now he reigns supreme as the World's Best in name and actuality, an exceptional master swordsman.”
The Databooks also stated that Mihawk :
"Trained relentlessly for many years challenging more and more powerful foes until he had no more. He then joined the Warlords"
Although hawk-eyes strikes fear into all who follow the way of the sword, he looks forward to the future, the day when he will meet a swordmaster that surpasses his rival Red-Hair".
One Piece Red: Grand Characters also described that Shanks’ swordsmanship is at top level, and that Mihawk is the No.1 swordsman.
So far, we hadn't seen the full power, skill, and abilities of Shanks and Mihawk, both characters displayed only a glimpse of their true strength.
Mihawk likes to test his foes and generally plays around. While Shanks only had a few clashes.
We know that Shanks still boasts an insane level of power, but it is now very difficult to know who is more powerful between Shanks and Mihawk.
Databook : "Even with one arm, his (Shanks) power is overwhelming".
This should be good enough.
He’s a Yonkou, one of the four Emperors of the Sea. Remember, it took the marines their entire military power to take down Whitebeard who was way past his prime. Imagine what would it take to bring down Shanks, and remember there are chances that Mihawk won’t be on the Marines side.
We know how strong Kaido is. Doffy, who almost defeated Law and Luffy, was scared of him. He took down Eustass Kidd, who’s supposed to be as strong as Luffy. He has man as strong as Jack as his underling.
A guy like Kaido was stopped by Shanks when he was chasing Whitebeard. And Shanks still managed to make it to Marineford.
There is still a lot of mystery around that scar,but we know some things about it and can make some conclusion based around it.
First thing is the time of scarring.
Shanks had that scar even before he met Luffy 12 years ago at Fyusha village.
Considering how Shanks introduced his scar to Whitebeard,saying that he got it from one of his men,not in fight between their crews,I think it is safe to say that he got it after Roger’s death,somewhere around 25–12 years ago from now.
We know that by the time Shanks came to Luffy’s villaige he was way beyond random pirate.
We don’t know if he was a Yonko level back then or even close,because he couldn’t just come of Roger’s ship being that strong as a cabine boy.
That means that he was probably not as strong as he is know,but strong enough to have daily duels with Mihawk that even Whitebeard praised,so he arrgument that at that time Blackbeard was stronger doesn’t hold water.
Second point is reason for scarring.
We don’t know much about that,but at that time Blackbeard was still on Whitebeard’s ship while Shanks was on his own.
Maybe some Whitebeard pirates went and had skirmish with Red hair pirates because of old times sake.
Let’s not forget that Blackbeard was in the second devision,and we know that he the position of commander of 2nd devision has been empty for some time.
Maybe that is somehow tied to everything.
Lastly is the type of scar.
As far as we can tell,Blackbeard gave Shanks that scar with Wolverine-like claw he used during his time as a Whitebeard pirate.
In One piece match-ups matter a lot,and for all we know,Blackbeard and Shanks situation might have been similar to Enel and Luffy situation,where one’s ability works great against another one,despite him being weaker overall.
Maybe that scar gave Shanks motivation to become a better with bladed weapon,who knows.
I hope I helped.
