One Piece is already in the final stages, which means only a select few end-game bosses can even touch Luffy's Gear 5.
The Straw Hat's ultimate transformation was first revealed in the climax of the Wano Country arc. He awakened his Devil Fruit to defeat Kaido once and for all. The Five Elders describe it as completely "ridiculous," since Luffy can fight any way he likes in the One Piece series.
With the events of the Egghead arc, Luffy can freely switch between his base form and the Gear 5 transformation. At this point, only a few One Piece characters pose a challenge. Here's a look at potential threats in no particular order. It's not a matter of whether or not they win, but rather if they can hold their own.
Note: This article reflects the writer's personal views. It will also contain manga spoilers that haven't been adapted to the anime yet.
The following One Piece fighters could realistically take on Luffy's Gear 5
1) Kaido
Kaido did in fact lose to Luffy's Gear 5 at the end of the Wano Country arc. However, keep in mind that he withstood multiple waves of strong enemies before this final encounter. Luffy was also completely worn out after the fight and fell into a coma for about a week.
It makes sense why Kaido is considered the "World's Strongest Creature" in the One Piece series. His physical stats are insanely high and he can dish out as much as he can take. Kaido still gave Luffy a very hard time towards the end of their climactic battle.
2) Aokiji
Aokiji commands a lot of respect for being a former Admiral. It's a prestigious title reserved only for the major powerhouses of the Marines. He is definitely up there in the top percentile of One Piece fighters.
All the original Admirals possess a Logia Devil Fruit, which gives them immunity to most attacks in the series. Aokiji has the ability to create and control ice with the Hie Hie no Mi. He could use it to freeze Luffy during his most chaotic moments, even if that doesn't last very long.
3) Charlotte Linlin - Big Mom
Big Mom is Kaido's equal in the One Piece series. However, she would be an even trickier opponent for Luffy. Her monstrous durability aside, Big Mom can also manipulate souls with the Soru Soru no Mi, which can be used to recover health and power herself up.
Luffy's Gear 5 gives him a wide variety of fighting styles as he basically turns into a cartoon. Not to be outdone, Big Mom has plenty of versatility with her Paramecia Devil Fruit, giving her powerful moves that range from natural disasters to elemental summoning familiars.
4) Kizaru
Kizaru is already heading over to Egghead by One Piece Chapter 1069, so he could have a potential showdown with the Straw Hat. The Marine Admiral might just be the fastest opponent that Luffy will encounter. Kizaru can move as fast as light itself with the Pika Pika no Mi.
Luffy is going to have to come up with some crazy plans while using Gear 5. Not only is Kizaru very difficult to hit, the Admiral can also launch several beams of light from a faraway distance.
5) Shanks
Right before Kaido was defeated, he told Luffy that Haki trumps Devil Fruits in the One Piece series. With that in mind, Shanks might have the strongest Haki so far, based on his showings in Wano Country. He could paralyze Ryokugyu from several miles away with a burst of Haoshoku Haki.
Shanks would definitely give Luffy an extremely difficult time if they fought. He spent several decades mastering Haki, so his skills should be finely tuned. While it's not confirmed, Shanks might also have background knowledge of Luffy's Devil Fruit, since he went after it a long time ago,
6) Akainu
Akainu is arguably the strongest of the three original Admirals, even if only by a little bit. Remember, he could withstand direct attacks from Whitebeard himself, a man who once held the title of strongest person in the world. Akainu can definitely tank Luffy's Gear 5 attacks.
It remains to be seen if Luffy can maintain his laughter against the man who killed his sworn brother. Sometimes One Piece fights can be as exhausting mentally as they are physically. It also doesn't help that Akainu has extremely high offense via the magma properties of his Magu Magu no Mi.
7) Marshall D. Teach - Blackbeard
Blackbeard is the perfect counter to Luffy's Gear 5 abilities in the One Piece series. With the power of darkness from the Yami Yami no Mi, he can directly cancel out Devil Fruit powers via physical contact.
Luffy will need to depend on his powerful Haki in this fight. He cannot make the same mistake that Blackbeard makes by overly relying on his Devil Fruit, especially if he cannot access those abilities.
8) Monkey D. Garp
Garp is among the heavyweight fighters of the One Piece series. He was up for promotion to the Admiral position multiple times, which at least puts him on the same level as the original trio. More importantly, Garp was also a rival to Gol D. Roger, the Pirate King himself.
The Monkey family is known for their extreme willpower in the One Piece series. Despite his old age, Garp still has the strength and stamina to fight his own son on relatively even terms.
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