NYC Mayor Bloomberg was No. 6 in Conde Nast Portfolio’s new generosity index, which listed 50 of the nation’s mega-rich from most to least giving, taking wealth into account.

Hizzoner, worth $11.5 billion, sits on a long list of nonprofit boards, from the New York Public Library to the World Trade Center Memorial Foundation. His pet causes are health care and the arts, and he gave $205 million last year.

“The mayor works for $1 a year and has made it clear that he intends to give away his money through charitable contributions,” said spokesman Stu Loeser.

No. 1 in the giving game was well-known philanthropist Warren Buffett, worth $52 billion. The Nebraska stock wizard gave an amazing $46.1 billion from 2002 to 2006, the magazine said.

And at the miserly end of the list at No. 50 – Rupert Murdoch. The 77-year-old News Corp. honcho, worth $8.8 billion, hasn’t disclosed any giving since 2002, the magazine said. But he has made $5,500 in political donations.

Now is the time for billionaires to help out, No. 22, Revlon owner Ron Perelman, told Portfolio.

“In times like this, when the economy is having difficulties, it’s more important for individuals to step up and do stuff privately that may have been historically funded by public money,” said Perelman, who gave $70 million in 2007 and 2008, according to Portfolio.

Many of the country’s wealthiest continue to be consistent givers. The magazine ranked Microsoft founder Bill Gates, known for his work with the Bill and Melinda Gates Foundation, at No.2.

One of the youngest billionaires at age 35, Google co-founder Sergey Brin was ranked No. 31. He founded the Brin Foundation when his company went public, making himself the president, his mother the secretary, and his father the CFO.

His business partner Larry Page, also 35, runs a foundation in memory of his father. He ranked 28th.