The Yonko, also known as the "Emperors of the Sea," are the four strongest pirates that rule the New World with unparalleled authority in One Piece. They're one of the three main superpowers in the series, with the other two being the Marines and the Shichibukai, both of which are there to counter-balance the Yonko. The strength of a Yonko is such that not even the Marines can take them on by themselves, which is why they need the Shichibukai. In the entire world of One Piece, there are few characters who are close to these monsters when it comes to strength, so we've compiled a list of those who are.
10 Boa Hancock
Also known as the Pirate Empress, Boa Hancock is one of the fiercest women alive in One Piece. She is a user of all three Haki types, which is a rarity in itself, but on top of that, her skills with her Devil Fruit, Mero Mero no Mi, make her almost impossible to defeat. Hancock possesses tremendous fighting capabilities, and just after her first voyage, a bounty of 80 million berries was slapped on her, and she was soon made a Shichibukai. Even when her status of Shichibukai was revoked by the government, she wasn't fazed in the slightest. Hancock may not be Yonko level, but she's as close to it as one can get.
9 Charlotte Katakuri
One of the three sweet commanders of Big Mom and Totto Land's Minister of Flour, Charlotte Katakuri is considered a legend who had never lost a single battle in his life until he crossed blows with Luffy. Just like Boa Hancock, Katakuri is a user of all three Haki types, and what's even more intriguing is that he's taken his Observation Haki to a level where it enables him to see the future itself. Yonko first commanders are next in line to lead the crew after the Yonko themselves, and an example of that is Marco the Phoenix. As such, Katakuri is close to the level of a Yonko, and might even reach the said target in the future.
8 Monkey D. Luffy
The protagonist of One Piece, and the man with a 1.5 billion berries bounty, Luffy is the captain of the Straw Hat Pirates. He's a tenacious fighter who was strong enough to defeat Katakuri in single combat. Luffy has grown at a blazing speed since the start of the series, and right now, he commands two advanced Haki types, as well as Conqueror's Haki. Luffy's Devil Fruit, called the Gomu Gomu no Mi, is also pretty close to awakening, and after being declared as the fifth Yonko, it is clear that he isn't too far behind the Emperors themselves.
7 The Three Admirals
The Admirals are considered the strongest force in the Marines, and their authority is next to only the Fleet Admiral. Under the current Fleet Admiral Akainu, the three admirals are Fujitora, Kizaru, and Ryokugyu. All three of the admirals are, more or less, the same in terms of combat strength, and while not exactly strong enough to defeat a Yonko, they are certainly close to their level. As demanded by their position, the Admirals possess two of the three Haki types, which are likely trained to their limits. On top of that, they also wield overpowered Devil Fruit powers, which range from controlling the gravity to transforming into light itself. The Admirals are indeed close to the Yonko.
6 Marco
Marco was the former first mate of the Whitebeard Pirates, and the right-hand man of the strongest pirate, Edward Newgate. Thanks to the astounding abilities of his mythical zoan Devil Fruit, Marco received the moniker of "Marco the Phoenix." After the passing of Whitebeard, Marco became the captain of the Whitebeard Pirates, and according to the Gorosei, he was one of the handful of people capable of defeating Blackbeard. Marco's authority as one of the strongest characters in the series is irrefutable, and he's certainly one of the few pirates who are close to the level of a Yonko.
5 Mihawk
The strongest swordsman in the world of One Piece, Dracule Mihawk, is unquestionably a rival to a Yonko in terms of strength. He is the wielder of one of the Saijo o Wazamono Grade Swords, known as Kokuto Yoru. Mihawk has impeccable command over at least two Haki types, and this assuredly makes him a threat to even the strongest of pirates. While not exactly as strong as the Yonko, he's sure to be on par with their strongest commanders, if not above. Mihawk has fought countless battles against Shanks back in the day, although back then, Shanks wasn't a Yonko.
4 Akainu
The current Fleet Admiral of the Navy, Akainu is a wicked man whose strength is considered absolute. Wielding the Devil Fruit called the Magu Magu no Mi, Akainu boasts an incredibly high offensive power, which is said to be of the highest order in the world of One Piece. As expected from a man of his caliber, he's also a proficient user of at least two Haki types. His authority is such that even with two Devil Fruits, Blackbeard didn't dare to face him. Akainu's powers are close to that of a Yonko, which was evident from when he faced off against Whitebeard at Marineford.
3 Rayleigh
Renowned throughout the world as the "Dark King," Rayleigh was once the right-hand man of Gol D. Roger. He made his appearance back in the Sabaody Archipelago arc of One Piece, and that's where he displayed a hint of his true power. Although old, Rayleigh is more than capable of fighting against an Admiral, and even injuring them. He doesn't possess a Devil Fruit, and thus, he utilizes Haki as his primary weapon. Rayleigh has trained his Haki to an advanced level, and that's what makes him a threat to the Navy, even when he's long retired as a Pirate. According to Garp, the Navy would need an all-out operation to take him down, which is identical to how the Yonko are dealt with.
2 Sengoku
Sengoku was the Fleet Admiral of the Marines, prior to Akainu. A legend in his own right, Sengoku is one of the most fearsome navy members in history. He's the only known Marine to possess Conqueror's Haki, and his command over the other two forms is resolute as well. At an unspecified time, Sengoku consumed a mythical zoan Buddha Devil Fruit that granted him the ability to transform into a giant Buddha, and emit shock waves. Before the great age of pirates, Sengoku clashed with several big names in Whitebeard, Shiki, and possibly even Roger at some point. Undoubtedly, Sengoku's slightly below the level of a Yonko, if not equal.
1 Monkey D. Garp
The man who famously cornered Gol D. Roger, Garp is a legend of the Marines, and quite possibly the strongest one at that. He's the man who, along with Gol D. Roger, put an end to Rocks. Garp is one of the most intimidating characters in the world of One Piece, and even at the age of 78, he's more than capable of taking on the top-tiers in the verse. He's a master of at least two types of Haki, but he specializes in Armament Haki's usage. In his prime, Garp's likely to have been on par with Roger himself.