Elvis Presley was known as "the King" of what?

The renowned singer and dominant performer Elvis Presley has had no shortage of nicknames, but his most memorable moniker is also his grandest: “the King of Rock ’n’ Roll,” or simply “the King.” Although its exact debut is unknown, that nickname made one of its earliest appearances on April 19, 1956, when reporter Bea Ramirez called Presley the “21-year-old king of the nation’s rock ’n’ roll set” in the Waco News-Tribune. Ironically, Presley himself declined that title: after his first opening in Las Vegas, in 1969, a reporter referred to him as “the King” in a press conference, which prompted him to point to Fats Domino at the back of the room and say, “No, that’s the real king of rock and roll.”

Source: Britannica