On May 12th of this year, Queen Elizabeth II of the United Kingdom became the second-longest reigning monarch in British history, passing King George III (a.k.a "Mad King George"). Only her great-great-grandmother, Queen Victoria, has had more days on the throne.
She is also second on the list of longest serving current monarchs. A similarly well-regarded king, who ascended to the throne about five years before her (he is a year younger to her), is on top of this list. Who and from which country?
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King Bhumibol Adulyadej (a.k.a King Rama IX) of Thailand.