NEW YORK ( TheStreet) -- What the financial crisis taketh away, it giveth -- if you're a billionaire, that is.

And especially if you're a billionaire named Carlos "Slim" Helu pictured below , who for the first time nabbed the number one spot on Forbes' list of the world's fattest cats.
Carlos Slim Helu
Carlos Slim Helu
Cratering financial markets last winter (banking crises, bursting asset bubbles, the severest economic downturn since the Great Depression) wiped out wealth on a torrential scale. On Forbes' 2009 list, Slim's net worth, for instance, plunged to $35 billion from 2008's bubble-inflated $60 billion, one of the steepest declines of any billionaire that year.

But last March, as everyone knows, the equities markets began what would become a roaring rally. The great bull bounce-back of the second half of 2009 had an obvious effect: it returned value to the paper wealth of the world's plutocrats, since most of the people on the list have vast holdings in the publicly traded companies they founded and in many instances still control.

>>Stockpickr Portfolio: The Richest People in the World

Slim -- the overlord, essentially, of the Mexican cell-phone system -- saw his fortune increase by $18.5 billion to $53.5 billion, according to Forbes' research, edging out perennial No. 1 Bill Gates by a measly half a billion dollars. (For this year's list, Forbes judged each billionaire's wealth as of Feb. 12.)

How did Slim do it? Not from his high-profile investment in The New York Times ( NYT), a move that foisted the self-made titan into the American spotlight, though the transaction certainly helped. Shares of the Times have more than tripled in value since last March, when the company was struggling under a heavy debt load and before Slim stepped in to assist.

His most important holding is the Latin American telecom empire he has built over the last 20 years: America Movil ( AMX). Since last March, its American depositary receipts have ripped higher by 58% -- buoying Slim's kitty until it's become the planet's most valuable, at least according to the reckoning of Forbes, which each year assigns a platoon of cub reporters to do nothing but dig into the books of the billionaires.

Gates, meanwhile, got a lift from his old company. Now the nation's foremost philanthropist, Gates has surrendered the top spot on Forbes' list only once in the last 15 years -- in 2008, when Warren Buffett took the crown, and Gates ranked third behind Slim. Shares of Microsoft ( MSFT) have rocketed 70% since last March. (According to Forbes, a little less than 40% of Gates' wealth is tied up in shares of the Redmond, Wash., company.)

And what of the Oracle, who fell to third on the list? Warren Buffett's net worth amounted to $47 billion, Forbes said, up from $37 billion in 2009 but still far less than the $62 billion stash Buffett sported at the peak of the bubble.

The folksy Nebraskan saw Berkshire Hathaway's ( BRK.B) class A shares lose half their value between September 2008 and March of last year. But a torrid first quarter of 2010 has helped that coveted equity increase by 70% over the last 12 months.

Steel baron Lakshmi Mittal has similarly benefited from an incredible 157% explosion in the share price (over the last 12 months) of the company he created four years ago, through a hostile merger: ArcelorMittal ( MT), the world's largest steelmaker. His personal fortune, meanwhile, has grown by 32% to $28.7 billion from $19.3 billion in 2009.

Other billionaires riding surges in the prices of their companies' shares: No. 6 Larry Ellison, whose hyper-acquisitive Oracle ( ORCL) has enjoyed a stock-price jump of 72% since last March.

Shop-keeping hasn't proved quite as lucrative, billionaire-investment-holdings-wise, as telecom, software or industrials. The affluence of the various Waltons on the list, for instance, (the richest are Christy, Jim, Alice and Rob) have increased, but not nearly as much as some of their monied peers.

Wal-Mart ( WMT) stock, after all, has gained just 13% since last March, a much worse performance than other stocks affiliated with Forbes-list tycoons.

-- Written by Scott Eden in New York

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Scott Eden has covered business -- both large and small -- for more than a decade. Prior to joining TheStreet.com, he worked as a features reporter for Dealmaker and Trader Monthly magazines. Before that, he wrote for the Chicago Reader, that city's weekly paper. Early in his career, he was a staff reporter at the Dow Jones News Service. His reporting has appeared in The Wall Street Journal, Men's Journal, the St. Petersburg (Fla.) Times, and the Believer magazine, among other publications. He's also the author of Touchdown Jesus (Simon & Schuster, 2005), a nonfiction book about Notre Dame football fans and the business and politics of big-time college sports. He has degrees from Notre Dame and Washington University in St. Louis.