Mr Slim's fortune rose by $18.5bn (£12.4bn) last year to $53.5bn.
That beat Microsoft founder Bill Gates ($53bn) into second place, with US investor Warren Buffet ($43bn) third. In 2009 332 names left the list after a tough year, but the total number of billionaires on this year's list rose from 793 to 1,011, Forbes said.
A spokesman for Carlos Slim refused to confirm the Forbes estimate of the Mexican tycoon's wealth, saying they did not "waste their time" on such calculations, but he welcomed the result.
"We are pleased that he's been considered the best businessman of the world," spokesman Arturo Elias told the BBC. "It means there is trust among the investors."
The year's biggest gainer, Brazilian mining tycoon Eike Batista, broke into the top 10 for the first time.
He came in at number seven, having boosting his wealth by $19.5bn to $27bn.
It is only the second time since 1995 that Gates has lost the crown, said the magazine.
Here is Forbes ranking of the 20 richest people in the world:
1. Carlos Slim (Mexico) - $US53.5 billion, telecommunications
2. Bill Gates (USA) - $US53 billion, Microsoft
3. Warren Buffett (USA) - $US47 billion, Berkshire Hathaway
4. Mukesh Ambani (India) - $US29 billion, petrochemicals, oil and gas
5. Lakshmi Mittal (India) - $US28.7 billion, steel
6. Larry Ellison (USA) - $US28 billion, Oracle
7. Bernard Arnault (France) - $US27.5 billion, LVMH
8. Eike Batista (Brazil) - $US27 billion, mining, oil
9. Amancio Ortega (Spain) - $US25 billion, retail
10. Karl Albrecht (Germany) - $US23.5 billion, supermarkets
11. Ingvar Kamprad & family (Sweden) - $US23 billion, Ikea
12. Christy Walton & family (USA) - $US22.5 billion, Walmart
13. Stefan Persson (Sweden) - $US22.4 billion, Hennes & Mauritz
14. Li Ka-shing (Hong Kong) - $US21 billion, diversified
15. Jim Walton (USA) - $US20.7 billion, Walmart
16. Alice Walton (USA) - $US20.6 billion, Walmart
17. Liliane Bettencourt (France) - $US20 billion, L'Oreal
18. S. Robson Walton (USA) - $US19.8 billion, Walmart
19. Prince Alwaleed Bin Talal Alsaud (Saudi Arabia) - $US19.4 billion, investments
20. David Thomson & family (Canada) - $US19 billion, news and information company Thomson Reuters
The full Forbes ranking of the world's billionaires can be seen atwww.forbes.com/billionaires.