An interesting ability in One Piece is the devil fruit system. Logia, Zoan and Paramecia are the three types of devil fruits. Zoans grant its users the ability to transform into some sort of animal. One typically has access to three forms; a normal form, beast form and hybrid.
All zoans provide users with a natural physical boost and increased animalistic instincts. Many of the strongest characters in One Piece have consumed as it is a sure-fire way to increase your strength even though you lose your ability to swim.
10 DRAGON-DRAGON FRUIT MODEL: SPINOSAURUS
This is one of the much-coveted dragon-type Zoan devil fruits and this alone goes to show why Page One became a member of Kaido's crew. It grants the user the ability to transform into a Spinosaurus, which grants an incredible boost in physical attack and defensive power. This is a carnivorous type Zoan, which is said to be the most suited for fighting. It is worth noting that this fruit grants the user particular strength in the tail and jaw, and has enough force to easily bite through buildings in Wano country.
9 DOG-DOG FRUIT, MYTHICAL TYPE, MODEL: NINE TAIL FOX
This fruit grants the user the ability to turn into a nine-tailed fox. By extension, this grants the user the ability to transform into other people. The fruit creates a seemingly perfect copy of the original user, meaning that one can't easily tell who is who, making the deceptive capabilities dangerous. Along with this, the user’s clothing mirrors that of the person they turn into. Additionally, the user has the same voice and speech pattern as the person being imitated. Catarina Devon was able to use this ability to completely trick Gecko Moriah a man who was close to the previous user of the fruit.
8 SNAKE-SNAKE FRUIT, MODEL: YAMATA NO OROCHI
This fruit allows a user to transform into the legendary Japanese serpent, the Yamata no Orochi. This is a fairly new devil fruit that debut in the Wano arc, but we've already been given glimpses on how powerful it could be. The elongated necks of the snakes mean the user has considerable range advantage to work with when attacking. Additionally, its size boosts, not only its defensive solidity, but also makes Orochi much more offensively destructive. The mouth of the snakes seems to be able to bite with significant power as well. It just goes to show that, if used properly, this could be one dangerous Zoan devil fruit.
7 DRAGON-DRAGON FRUIT, ANCIENT MODEL: ALLOSAURUS
Red flag X Drake consumed this devil fruit and, with its ability, he's become a prominent member of Kaido's crew, while working undercover for the marines. His fruit allows him to transform into an ancient form of dinosaur known as an Allosaurus. This provides the user with incredible biting force, as well as the instincts of one of nature's most deadly predators. The user of the ability also regains stamina and heals at an incredible rate. Before the time skip, while using this ability, Drake was able take down a pacifista.
6 DRAGON-DRAGON FRUIT, ANCIENT MODEL: BRACHIOSAURUS
Queen, the plague, consumed the ancient zoan devil fruit that gave him the ability to turn into a Brachiosaurus. With this ability, he amassed enough power to gain a bounty of over a billion berries and has become a top commander. Queen is absolutely huge in his transformed state and this makes him extremely physically sturdy. He's adept at using his weight and massive size to generate power in his attacks. This is a sensible way of fighting with a devil fruit that grants such incredible size.
5 ELEPHANT-ELEPHANT FRUIT, ANCIENT MODEL: MAMMOTH
As the name suggests, this fruit allows to user to transform into a mammoth, which grants the user incredible size and power. Jack has been said to be able to stomp through countries and leave it looking as if it was hit by a drought. That is due in part to the incredible size of his mammoth form, as well as the stomping power. The Mammoth fruit's defensive abilities were enough to handle fighting both guardians of Zou. Additionally, the mammoth's trunk allows for powerful swinging attacks that can easily destroy buildings. The Mammoth’s incredible size does make it somewhat slow, so users can be susceptible to faster fighters.
4 HUMAN-HUMAN FRUIT, MYTHICAL TYPE: MODEL BUDDHA
Another Zoan devil fruit that grants the user into an incredible size in order for a massive power upgrade. This Zoan is quite unique as it grants the user additional abilities in conjunction with the regular zoan abilities. The user of the Buddha fruit gains the ability to launch shockwave type attacks at opposing forces. Sengoku, The Buddha, consumed this fruit and throughout his time as admiral, he's used it to bring justice to the world. This fruit's abilities were on show at Marineford, where Sengoku used it to defend Marineford against Blackbeard.
3 DRAGON-DRAGON FRUIT, ANCIENT TYPE: PTERANODON
King is one of Kaido's calamities and is known to bring destruction anywhere he goes which has led to him rocking up a bounty of over a billion berries. King's devil fruit allows him the ability of flight which is dangerous anywhere you wish to do battle. He was also big and strong enough to knock back Big Mom's flagship when she tried entering Wano. The massive wingspan means that a user can also be used as a means of transportation when the situation arises.
2 BIRD-BIRD FRUIT, MYTHICAL: PHOENIX
A unique devil fruit even amongst the mythical Zoan category. The Bird-Bird fruit model, phoenix allows the user the ability to generate 'Flames of Resurrection', which grants this user the ability to heal from damage. This devil fruit provides the ability of flight, along with an increase in other physical abilities and is known to be particularly durable, even amongst Zoans. This fruit has aided Marco, not only in fighting, but also as a doctor. These flames of resurrection ensure that users who don't wish to fight can settle for being miracle healers. These flames do not provide heat, but the restorative properties means that it is just as useful as destructive fire.
1 KAIDO'S DEVIL FRUIT
Kaido consumed a so-far unnamed a mythical zoan devil fruit that granted him the ability to transform into an eastern dragon. With this ability, he is able to simulate flight by summoning stormclouds and climbing onto them. Kaido is commonly thought of as the strongest creature alive and his dragon scales are considered impenetrable. It is not known when Kaido consumed this fruit, but it has surely added to what is a deadly arsenal at his disposal. Kaido's heat breath was more than enough to easily obliterate a mountain when he attempted to attack Luffy in Wano.