The Guinness Book of Records (2013) has named King Bhumibol of Thailand as the world’s wealthiest monarch. This is what they say:
Richest monarch According to Forbes, as of July 2011, the wealthiest monarch is Thailand’s King, Bhumibol Adulyadej, King Rama IX of the Chakri dynasty. Although his wealth declined by some $5 billion in the year following the global financial crisis of 2008, he is still managing to get by, with an estimated fortune exceeding $30 billion, much of which stems from investment in Thai businesses.
The Thai King also holds a record for the longest reigning monarch at 65 years. This is followed by Queen Elizabeth at 60 years.