Which US president was nicknamed "King of Camelot"?

The term was actually assigned by Jacqueline Bouvier Kennedy (1929–1994). Shortly after President John F. Kennedy was assassinated (November 22, 1963), the former First Lady was talking with a journalist when she described her husband’s presidency as an American Camelot, and she asked that his memory be preserved. Camelot refers, of course, to the time of King Arthur and the Knights of the Round Table, and has come to refer to a place or time of idyllic happiness. Despite subsequent revelations that there were difficulties in the Kennedy marriage, public opinion polls indicate that the image of Camelot—albeit somewhat tarnished—has prevailed.

Source: The American Presidency