Marines are the second-highest authority in the One Piece universe, behind the World Government. They are renowned for having some of the most talented fighters in the entire globe among their ranks. The Marines are responsible with keeping order on all known seas in the world, which they attempt to do through a vast network of branch offices. High-ranking Marines like admirals and fleet admirals are physically capable of fighting Yonko head-to-head. They are, nevertheless, viewed as one of the series' key antagonists as a result of their acts.
Until the Seven Warlords of the Sea were replaced by the SSG, the Marines, Yonko, and the Seven Warlords of the Sea jointly made up the Three Great Powers that uphold the world's order. The Marines who actually stand out among the many powerful Marines include Garp, Akainu, Sengoku, Kizaru, and retired Admiral Aokiji. Among competitors for the title of strongest Marine are the recently appointed admirals Fujitora and Green Bull.
How Durable Are the Marines Overall in One Piece?
Each admiral and fleet admiral has strong Devil Fruit powers. Akainu has the ability to create, control and transform into magma at will. Kizaru can create, control and transform his body into light. Fujitora can manipulate gravitational forces and even bring down meteors. Sengoku, on the other hand, has the power of a mythical Zoan-type Devil Fruit that allows its user to transform into a Daibutsu, a giant gold Buddha.
Even without these Devil Fruit powers, each one of them is exceptionally strong with great control over their Haki. However, Garp has no Devil Fruit but is exceptionally talented in Haki.
The Series' Most Powerful Marine is Garp
Although he is only a vice-admiral, Garp is the strongest Marine in the series, surpassing even the likes of Sengoku and Akainu. It's a common misconception that Sengoku is stronger than Garp because the former is able to hold him down during the Summit War and stop him from attacking Akainu. However, the truth behind it was that Garp intentionally let Sengoku do this so he wouldn't end up killing the man who killed his grandson. Had it not been for Sengoku, Garp would have easily overpowered Akainu.
Garp's physical strength is so immense that he could easily defeat Marco, a mythical Zoan user, with a single blow. The only reason Garp refuses to become an Admiral is that he despises the Celestial Dragons -- a fact that he never hides from anyone. Although showing disrespect toward the Celestial Dragons is a serious offense, Garp is always pardoned because of his unparalleled accomplishments. He is the only Marine capable of going against the Pirate King himself and holds the title of "hero."