Microsoft chairman Gates, a perennial top finisher in the list, placed second with $67 billion, up $8 billion from 2012.
Ortega, whose Inditex fashion group includes the popular Zara chain, vaulted from the seventh position into the third spot. His wealth is now estimated at $57 billion, rocketing from $31 billion a year ago.
Slim and Gates were also in the top two spots last year.
Warren Buffett, chief of US conglomerate Berkshire Hathaway, ranked fourth with $53.5 billion, and Larry Ellison, chief executive of US technology company Oracle, rated fifth with $43 billion.
Asia's richest man, Li Ka-shing, came in eighth with $31 billion. He owes to fortune to his sprawling transportation, trading and energy businesses.