In East Blue, Oda highlighted the scarcity of devil fruits and how valuable they are. In the first half of the Grandline, devil fruits got a bit more regular, albeit not as common. In the second half of the Grandline, the New World, devil fruits became as common as grains in a field-- even cannon fodders had the privilege of eating one.
With this sudden influx of devil fruits, many seemingly forgot how terrifying these things could be. A good chunk of the devil fruits either had their potential strangled by their incompetent users, or they were just cruel victims of the One Piece narrative. That being said, not every devil fruit has the potential to conquer all; even though some actually do.
10 Rust-Rust Fruit
Rust-Rust Fruit is the nemesis of swords and all metallic weapons for that matter. This fruit enabled Shu, a Marine captain, to easily destroy one of Zoro’s swords and nearly defeat the man in question.
If Shu, alongside his devil fruit, got enough attention from Oda, he would’ve undoubtedly made it to the frontline of the series. With a devil fruit so broken, he has to make it to the frontlines.
9 Kanjuro’s Devil Fruit
Oftentimes, devil fruits don’t get to shine due to a deliberate misuse from the user, and when they finally get their opportunity to shine, we naturally get bamboozled. This was the case with Kanjuro and his mysterious devil fruit.
If you are aware of the recent manga events, then you’d know the immense potential slumbering within this devil fruit. Its only limitation is the user’s own imagination.
8 Flower-Flower Fruit
As it is, the Flower-Flower Fruit is already quite notable. That being said though, the fruit would be capable of much more if Haki -both Armament and Observational- were added to the equation.
With Observational Haki, Robin would be able to extend her hands to locations that would usually be impossible to pinpoint through normal means, such as internal organs.
7 Slow-Slow Fruit
The fate of the Slow-Slow Fruit was more than just tragic. Not only was its user unimaginably incompetent, but the devil fruit itself was only featured for comical purposes. It’s the worst fate any devil fruit could suffer; let alone a devil fruit with an immense potential like the Slow-Slow Fruit.
The ability to drastically slow time for an opponent can prove deadly in most case scenarios. Even a Yonko would find it troublesome if it were to be used in the right context.
6 Weapon-Weapon Fruit
The Weapon-Weapon Fruit is yet another devil fruit with limitless possibilities. The term ‘weapon’ is so general that it could be applied to just about anything with the capability of causing harm. And Baby 5 can transform a part of, or her entire body, to the said weapon.
With an imagination creative enough, this devil fruit could pull impossible stunts. Stunts such as morphing into a nuclear weapon.
5 Kid’s Devil Fruit
So far, we have no confirmed information regarding Kid’s devil fruit and its full capabilities. But from what we’ve witnessed, there seem to be two prominent routes which the devil fruit could follow: the simple magnetization route and the complex electromagnetic field route.
If the fruit was simple, then that would be the end of it. If it was electromagnetic field-driven, however, then it could quite possibly be one of the series’ most overpowered devil fruits. To control the electromagnetic field is to control electricity, humans...etc
4 Bomb-Bomb Fruit
Considering how early on this devil fruit made its debut, we can pretty much deduce why it was nerfed. A Bomb-Bomb Fruit not restricted by the system is not a devil fruit that can be handled by the pre-time skip Straw Hats.
Essentially, this fruit allows its user to detonate every part of their own body with zero consequences whatsoever; effectively rendering them a walking bomb.
3 Rubber-Rubber Fruit
The Rubber-Rubber Fruit is a prime example of how a seemingly useless devil fruit can have such immeasurable potential. Furthermore, it also supports the argument that there’s no such thing as a useless devil fruit, only useless users.
With that in mind, it took Luffy more than a decade to master the Rubber-Rubber Fruit to such an extent, and he has yet to fully master it.
2 Op-Op Fruit
True to its name, the Op-Op Fruit is just downright overpowered. But even with how overpowered this devil fruit is, there’s still much more left to unpack. Given enough growth, it could even gain an undefeatable status.
Presumably, this all comes to whether Law awakens it or not. If he does awaken the Op-Op fruit, then he would be able to give Luffy a good run for his money. Even the almighty Yonko would find it hard to compete with that kind of Law.
1 Tremor-Tremor Fruit
Currently, the Tremor-Tremor Fruit stands at the top of the devil fruit hierarchy. It’s the uncontended king of paramecia. According to Marine legends -Garp and Sengoku-, it has the power to destroy the world.
There likely exists no devil fruit to match up the pure badassery of the Tremor-Tremor Fruit; one of the few devil fruits befitting the previous World’s Strongest Man. Now that it's in Blackbeard's possession, its fate remains unknown.