Cuban President Fidel Castro was furious when Forbes magazine estimated his fortune at $US550 million ($A714 million) last year. This year, the magazine upped its estimate of the communist leader's wealth to a cool $US900 million ($A1.2 billion).

Castro, who says his net worth is nil, is likely the beneficiary of up to $US900 million ($A1.2 billion), based on his control of state-owned companies, the US financial magazine said in its annual tally of Kings, Queens & Dictators fortunes on Thursday.

Kings and sheikhs of the oil-rich Gulf Arab states still top the Forbes list, to be published in its May 22 edition.

Saudi King Abdullah is number one with an estimated $US21 billion ($A27.3 billion), followed by Sultan Hassanal Bolkiah of Brunei at $US20 billion ($A26 billion) and United Arab Emirates' President Sheikh Khalifa bin Zayed al-Nahayan at $US19 billion ($A24.7 billion).

Among Europeans, Prince Hans-Adam II of Liechtenstein improved upon his family fortune of palaces, real estate and artwork with an investment in a US producer of hybrid rice, for total estimated riches of $US4 billion ($A5.2 billion).

Perhaps the most industrious of the leaders listed is Sheikh Mohammed bin Rashid al-Maktoum, ruler of Dubai, with a net worth of $US14 billion ($A18.2 billion).

Forbes estimates the renowned racehorse breeder also helped raise Dubai's gross domestic product from about $US8 billion ($A10.38 billion) to nearly $US40 billion ($A52 billion) since 1994 by diversifying its industries outside of oil and making successful investments overseas.

"He would probably be the shrewdest of the bunch," said Luisa Kroll, associate editor at Forbes.

Africa's Teodoro Obiang Nguema Mbasogo, president of Equatorial Guinea, made the list of wealthiest leaders for the first time. He is estimated to hold up to $US600 million ($A778.9 million), the magazine said, although an oil boom has not prevented his country's slide down the United Nations' development rankings.

Castro had said he was considering suing after Forbes released its 2005 list, scoffing then his wealth was estimated to be close to that of Britain's Queen Elizabeth.

"Do they think I am (former Zairian President) Mobutu (Sese Seko) or one of the many millionaires, those thieves and plunderers that the empire has suckled and protected?" he said last year, referring to his capitalist archenemy, Washington.

This year, Castro would be well above the British monarch. Queen Elizabeth came in with some $US500 million ($A649.1 million) in estates, gems and a stamp collection built by her grandfather. The list does not include Buckingham Palace or the crown jewels.

A copy of the list, compiled by Forbes editors and not confirmed by the royals themselves, was released on Thursday.

"People are always intrigued. What is the ultimate fantasy but being a rich princess or prince?" said Kroll, who edits the magazine's annual list of global billionaires.

"We keep it separate from the billionaires because there are some very tricky things about these folks," Kroll said. "It's very hard to separate state from personal wealth. Some of these fortunes literally go back 800 years."

© 2006 Reuters