“Dark King” Silvers Rayleigh is the former first mate of the Roger Pirates, known as the “Right Hand of the Pirate King”. He’s extremely powerful, easily one of the strongest characters introduced.
Shakky vouches for his strength by saying that he is “100 times stronger than all of you boys”, referring to the Eleven Supernovas.

When Garp was informed Rayleigh may be on Sabaody Archipelago, he told his men not to confront him, he reasoned that the Marines should not confront two legends, Whitebeard and Rayleigh at the same time.

Due to his phenomenal strength, Rayleigh was able to fight Admiral Kizaru equally and he even managed to scratch the admiral. However, he himself has admitted that he is not as young as he used to be, and can tire in battle. This may mean when he was in the prime of his strength he may have been even stronger than when he matched Kizaru, a true testament to his strength!

Jinbe, a former Shichibukai who was used to seeing Whitebeard, was later shocked to see Rayleigh in person. Eustass Kid himself even states that Rayleigh was “a Legend”.

Rayleigh has tremendous physical strength despite his elderly age. He is powerful enough to kill several Sea Kings with his bare hands while swimming in the Calm Belt.

Furthermore, Rayleigh is one of the very small number of people in the world who can use all three types of Haki. He is shown to possess great mastery of Haki and made use of all three types while teaching Luffy about them, and training him in the basics.

He has mastered Conqueror’s Haki to the extent that he can render a specific target completely unconscious and is able to coat himself with it.

Rayleigh has an extremely high level of skill in Armament Haki, allowing him to effectively bypass the powers of a Devil Fruit user and touching the “substantial body” beneath whatever protection the fruit provides as seen when he used Busoshoku Haki to kick and redirect an attack from Admiral Kizaru.
He can utilize an advanced application of Armament Haki which enables him to emit his Haki a short distance without a medium.
He is also able to use a high-grade version of this technique, where Rayleigh can channel his Haki into his opponent, letting him destroy them from the inside out, which he first used to destroy Camie’s collar.

Observation Haki can allow Rayleigh to sense the emotions and nature of others. The strength and mastery Rayleigh has over this kind of Haki is also shown by the fact that he can sense people all over the island he is currently on while most people can sense only in their immediate vicinity.

The “Luffy Gear 4 Vs Rayleigh” filler scene in Episode 870 reminded us that Rayleigh is still one of the strongest beings in the entire One Piece World. Luffy Gear 4 Bounceman was just a punching bag to Rayleigh. He probably could have one shotted Luffy just like Kaido did if he was a bit more serious. This could mean that Rayleigh is close to Yonko level if not Yonko level!

Rayleigh made a very prominent return in Egghead Arc, which is why fans are hyped for him. He is strong enough to convince Blackbeard to get away from Amazon Lily.
However Rayleigh himself admits that they were lucky, as it was his reputation that ended the conflict, and that he won’t be able to beat Blackbeard head-on due to his old age.