I believe he is the strongest firstmate outside Rayleigh of course
Marco is someone highly confident in his potential abilities to the point of
1- Talking down to even a yonkou like Shanks:
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2- Joking around admiral attacks:
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3- Being full confident he could rescue Ace by himself despite being guarded by a fleet admiral(Sengoku) and a legendary marine(Garp):
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WB himself trust his power enough to:
1- Give him full responsibility in protecting Luffy against any enemy in marineford:
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The Gorosei themselves:
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1- Put him in the same breath with yonkous
2- Highly confident in his strength to the point they think he is the only one besides Yonkous themselves who can take down Teach
And yes i know they said Marco and the remnants Whitebeard, but it's the same with Yonkou and Teach. When they are mentioned, it's obviously not just the captains. Marco was under WB on the otherside, so they couldn't have said and Yonkou Marco or the Marco pirates. It's for that reason they said Marco and the remnants Whitebeard pirates.
Marco's name was clearly spoken separately from the rest of the crew, indication that he is the strongest & most important member. They considered him specifically out his group & made the comparison. Just like you would say Shanks and the Red Hair, Big Mom and the Big Mom pirates and now Marco and the WB remnants and so on.
It's a crew vs crew in globality but with the captains as main factors. Even after losing their captain, their 2nd division commander, their 3rd division commander being permanently crippled, & overall their entire crew being severely crippled, distraught, and emotional torn after battling the entire force of the WG, they were still compared to the other yonkou with Marco as their leader
Feats
Kizaru couldn't make him tangible while using his strongest attack to date(yasaka no magatama). Marco on the other side while shrugging off Kizaru's attacks as if nothing happened, flew in and appeared above Kizaru before he even had time to realise it, forced Kizaru into a guarded position, and actually break through his guard with sheer force, sending him crashing to ground from an insane distance in marineford at an incredible speed:
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After which, Kizaru even warned the other marines to be weary of Marco. He also came at Aokiji with enough power, he broke through his weapon and still had enough force to send him flying:
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This let's you know he is at least on par with the admirals when it comes to physical strength if not above. He is a zoan after all(and a mythical one). The best fruit when it comes to physical strength boost
In terms of endurance/stamina, he shrugged off a heavy punch from Garp, tanking multiple lasers in back from Kizaru even while under the effects of sea-stone(even a blast right through the chest):
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, and showed no signs of slowing down even till the end of the war. One of those lasers alone was enough to take down supernovas in Shabondy. Multiple of them don't take down Marco despite letting his guard down, being under kairoseki(so impossibility to regenerate) and being heavily restricted
