NEW YORK (AP) _ Bill Cosby may face a television challenge from the Simpsons cartoon family this fall, but he leads all entertainers in the money-making ratings, according to Forbes magazine.

Cosby topped the business magazine's list of the world's 40 richest entertainers with $115 million in estimated gross earnings for 1989 and 1990. The magazine said Cosby made $60 million last year and projected he would earn $55 million this year.

In its Oct. 1 issue, released Monday, Forbes said the 53-year-old Cosby is making an estimated $4 million a month on syndicated reruns of ''The Cosby Show.''

Pop singer Michael Jackson, who hasn't released an album or gone on tour this year, fell to second place after holding the top spot for two years. Forbes said Jackson had two-year earnings of $100 million, $35 million of that in 1990.

Jackson's singing sister, Janet, was a newcomer to the list, ranked 26th with estimated two-year earnings of $30 million.

The Rolling Stones rock group - with $88 million - rose to third place from eighth last year thanks largely to a big concert tour. Movie producer-director Steven Spielberg, whose ''Arachnophobia'' was a box-office success this summer, made $87 million to slip to fourth place from second.

New entrants on the list were the teeny-bopper sensation New Kids on the Block in fifth place. Forbes said the group would make $78 million in the two years, $61 million of it in 1990.

Forbes said the estimated gross incomes were based on talks with industry executives, lawyers, agents and managers. It said earnings figures for recording artists are based on royalty payments from record and video sales and estimated pretax gross for concert tours.

Rounding out the top 10 were television personality Oprah Winfrey, $68 million; actor-producer Sylvester Stallone, $63 million; pop singer Madonna, $62 million; bodybuilder and movie star Arnold Schwarzenegger, $55 million; and Peanuts artist Charles M. Schulz, $54 million.

Newcomers to the list included former Beatle Paul McCartney, with an estimated income of $45 million, thanks to a yearlong concert tour; actor Bruce Willis, star of the two ''Die Hard'' movies and of the December-release ''Bonfire of the Vanities,'' based on the Tom Wolfe novel, at $36 million; and actor Sean Connery, who starred in the film ''The Hunt for Red October,'' at $35 million.