“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.