The Yonko are known to be some of the strongest characters in the world of One Piece. They are the four mightiest pirates of their age who sit atop the era and rule the seas of the New World like Kings. The power of the Yonko is feared by even veteran pirates and high-ranking Navy officials alike.
The Yonko are not pirates to be messed with and not even someone as powerful as a Marine Admiral can simply engage them on a whim. Given that the Yonko sit at the top of the pirate era right now, there are only a handful of characters who have, throughout history, surpassed this level and become powerful enough to be declared stronger.
5 Monkey D. Garp
Garp is one of the strongest characters in the world of One Piece and, despite being a free-spirited person who enjoys his freedom the most, he serves the Navy. He is exceptionally powerful, known to be the strongest member of the Navy in history. In fact, he even managed to go toe to toe with the Pirate King, Roger, which just goes to show how strong he truly was during his prime.
Even in his old age, Garp does not hesitate to fight against the likes of the Yonko, as evident by the fact that he did not even think twice before attacking Blackbeard's island. Garp possesses the power to use all three types of Haki and his Conqueror's Haki enhances his punches to the next level. Someone who could fight on par with Roger is certainly above the current Yonko.
4 Gol D. Roger
Gol D. Roger was the man who conquered it all – wealth, fame, and power, he had a firm grasp on everything by the end of his pirating career. Roger was, in many ways, just like Luffy. At his heart, he was like a child, however, when needed, Roger could turn into a demon as well. He had tremendous power despite not being a Devil Fruit user.
His Haki was extremely overpowered, as confirmed by Kaido himself. Despite being sick, Roger could fight against Whitebeard and tie the match, which just goes to show that he was in an entirely different league. Only a handful of characters can ever hope to stand up against him and the Yonko certainly don't have the same pedigree.
3 Rocks D. Xebec
Rocks D. Xebec is known to have been the most formidable enemy that Gol D. Roger ever faced. This statement in itself means that Rocks was a bigger rival to Roger than even Whitebeard. Rocks captained the Rocks Pirates and he had established a mighty group that included three of the future for Yonko in Charlotte Linlin, Kaido, and Edward Newgate.
When it comes to power, Rocks was a complete monster, strong enough to make the entire world fear him. He ruled the seas all by himself back during his age and the Government even declared him a terrorist, based on how he was acting. Rocks met his end and God Valley, 38 years ago. To defeat him, garb and Roger had to combine their forces, would just goes to show that he was in a league of his own and also the mightiest pirate that the world had possibly ever seen.
2 Five Elders
The Five Elders are the highest-ranking Celestial Dragons in One Piece that the people of the world know of. They are also the highest authority in the entire world, responsible for running the entirety of the Government. They enjoy the most power and, as such, they don't really step into action often, or ever. However, that doesn't mean that they aren't strong. The Gorosei are all incredibly powerful, as was confirmed in the recent One Piece chapters.
All of them seem to possess Mythical Zoan Devil Fruits. On top of that, they are the personal bodyguards of Imu, which means that their power must be on an entirely different level when compared to the rest of the strong fighters in the world. The Gorosei are certainly in a league of their own and, given that their authority is much higher than that of a Navy Admiral, they are also much stronger.
1 Imu
Imu is the sovereign of the entire world and quite possibly the most powerful character alive right now. They appear to be a relic from the Void Century yet, they are still alive, likely due to their immortality, which they somehow attained. Imu is also a Zoan Devil Fruit user, which is likely going to be the mythical type. For now, fans do not know what powers they have, however, they were able to eat Sabo's fire with rather ease.
At the same time, Imu also possesses powerful weapons at their disposal using which they can wipe out entire countries within a matter of a second. Surely, their true power is frightening and fans have not even seen it in full. Yet, what little Imu has shown so far is enough to place them on a level above the Yonko, especially given the firepower that they have at their disposal.