While Monkey D. Luffy is officially recognized as one of the Four Emperors (Yonko) in the One Piece story, the debate over whether he is the "weakest" among them is a major topic within the community.
Yes, Monkey D. Garp can still fight on par with Marines Admirals even past his prime, a fact demonstrated by his performance against Aokiji Kuzan on Hachinosu Island.
Admiral Kizaru, AKA Borsalino, lacks the sheer offensive strength of the Monkey family, but the eater of the Pika Pika no Mi is still the superior fighter overall.
The first Marine Admiral fans were ever introduced to, Kuzan now serves as the strongest of Blackbeard's Ten Titanic Captains.
The world's strongest swordsman, Dracule "Hawkeye" Mihawk, is one of several major One Piece characters who have been a part of the anime since the beginning, but who fans have still never seen in a full fight.
Yes, Fleet Admiral Akainu (Sakazuki) is widely regarded as having dealt the fatal, killing blows to Edward Newgate, known as "Whitebeard"—the man recognized as the Strongest Pirate of the Modern Era during the Marineford Arc.
One of the most overpowered characters in One Piece, there's a reason the fandom universally agreed Blackbeard, AKA Marshall D.
Introduced in the currently ongoing Elbaf Arc, Loki was established to be a monster when he was first introduced, but recent chapters have made it apparent that almost no one in the world of One Piece is stronger than he is.
The only one of the original four Emperors of the Sea still standing, Shanks is far and away the strongest mortal character in One Piece.
The dark counterpart of Sun God Nika, the likely slayer of Joyboy, and the oppressive, immortal ruler of the world, Imu is unquestionably the strongest character in One Piece.