Luffy is one of the strongest characters in One Piece right now and, after the events of Wano, he recently ascended to the throne of Yonko. Luffy is considered to be an extremely dangerous pirate. His Straw Hat crew is feared all over the world.
While their beginnings were certainly humble, they have become a force to be reckoned with. After all their years on the seas, only a handful of characters can ever hope to stand a chance against them in battle. Luffy, especially, is nearly unbeatable in combat, however, there are a bunch who might be able to pull this task off.
6 Shanks
Shanks is one of the Four Emperors of the Sea, just like Luffy himself. He is an extremely powerful pirate who relies on swordplay and Haki in battle. Shanks is the apprentice of the former Pirate King, Gol D. Roger and his battle prowess is certainly to be feared. In the recent Egghead arc, Shanks clashed against Kid and wiped out his entire crew in a single swing of his sword, despite their captain having a 3 billion berri bounty, which is equal to that of Luffy.
This just goes to show that Shanks is on a different level altogether. While Luffy is certainly much stronger than Kid and would not lose with a single swing of the sword, if Shanks does go allowed in an extended battle against Yonko Luffy, chances are, that he will win.
5 Blackbeard
Blackbeard is one of the main antagonists of One Piece and an extremely powerful pirate in his own right. He ascended in the world of piracy after stealing the Yami Yami no Mi from Teach, the former Fourth Division Commander of the Whitebeard Pirates. After escaping from the crew, he marked the beginning of the Blackbeard Pirates and, in years, this crew has now become one of the Emperor crews of the One Piece world.
Blackbeard also stole Whitebeard's Gura Gura no Mi and this fruit, combined with his darkness powers, makes him nigh invincible. He was able to defeat Ace before the 2-year time skip and, after the timeskip, he has only gotten much stronger. He recently clashed with Law after the events of Wano, and this fight will likely end in his favor as well. Given that he can nullify Luffy's powers as well as use the earth-destroying quakes, Blackbeard can certainly defeat Yonko Luffy.
4 Dragon
Luffy's very own father, Dragon is the head of the Revolutionary Army and also the World's Worst Criminal. Given the fact that he is the worst criminal in the world, his bounty is bound to be extremely high. Dragon commands a powerful force that is strong enough to rival the military forces of the Celestial Dragons. That is the vow he took months after the day Ohara was destroyed.
He has successfully established a mighty organization and, given that his subordinates are powerful enough to fight the Admirals, Dragon himself must be a level above. Furthermore, Dragon is from the Monkey D family and fans already know the legendary status that Garp holds, as well as Luffy right now. It only makes sense for Dragon to be extremely overpowered, especially now that he is in his prime. He would certainly be capable of defeating Yonko Luffy.
3 Sakazuki
Sakazuki is one of the main antagonists of the One Piece series and a character who not many fans give the deserved credit. He was formerly an Admiral of the Navy and a character who most fans despise due to the events of the Paramount war. That said, Sakazuki is what a great villain should be – a proper thorn in the path of the main character. Sakazuki hates pirates and believes in Absolute Justice. For this reason, he has changed the way the Navy works and is actively challenging the Yonko in the New World.
Luffy is no exception and Sakazuki will eventually clash against him and his crew, if given the chance. It must be remembered that he is an exceptional fighter who possesses the Logia type Magu Magu no Mi, with the powers of the highest order. At the same time, the former Fleet Admiral also had access to Conqueror's Haki, so it wouldn't be a surprise to see Sakazuki wield this skill as well.
2 Gorosei
The Gorosei, known as the Five Elders, are five mighty people who are the highest authority within the World Government that the people know of. They are the highest of the Celestial Dragons and their influence in the world is legendary. While it isn't known if they are fighters just yet, chances are that they possess battle capabilities and this is suggested by the fact that one of them wields what could be the Shodai Kitetsu, one of the 12 Supreme Grade Swords in the One Piece world.
One of them, namely Saint Jaygarcia Saturn, has recently descended upon the Blue Sea and he is currently heading to Egghead Island, where he will undoubtedly come face to face with Yonko Luffy, who is also one of his biggest enemies.
1 Imu
Imu is the highest authority within the World Government and also the entire world of One Piece. Their existence is completely hidden at this point and only the Gorosei, along with a select few individuals, such as Sabo, know of their existence. Imu is likely very powerful and they also seem to possess what could be an Ancient Weapon at their disposal.
Using its powers, they were able to wipe out the Kingdom of Lulusia within a matter of seconds. As such, Imu can kill Yonko Luffy if they so wish. For now, Imu and Luffy have not come face to face, however, eventually, they will meet and it will certainly make for an interesting showdown.