Elizabeth II
Elizabeth became queen when her father, King George VI, died on 6 February 1952. At the time of her death, she had ruled longer than any other king or queen in the history of the United Kingdom. She had also ruled longer than any other living king or queen in the world, since the death of Thailand's Bhumibol Adulyadej on 13 October 2016. She was the second-longest reigning monarch in history.