Michelle, 46, ranked number one in the overall list of 'The World's 100 Most Powerful Women' 2010 released today. She ranked no.1 in 'political' category too.
Kraft Foods Chief Executive Irene Rosenfeld ranked second and is followed by media mogul Oprah Winfrey in the third spot and German Chancellor Angela Merkel who ranks fourth.
Nooyi, the India-born chief executive of Pepsi, ranked 6th on the list.
Two other Indian women -- Axis bank Chief Executive Shikha Sharma and ICICI bank head Chanda Kocchar also make it the list.
Sharma is ranked 89th in the list while Kocchar comes in at the 92nd spot.
Forbes has divided the power women candidates into four groups: politics, business, media and lifestyle. It ranked the women in each group, and then group against group.
The 2010 World's 100 Most Powerful Women list "reflects the New Order of now," Forbes said, adding that its assessment is based less on traditional titles and roles and more on creative influence and entrepreneurship.
"These power women have built distinctive companies and brands and championed weighty causes, sometimes through unconventional means; in other cases they have broken through gender barriers," it said.
Calling Michelle "a true change-maker", Forbes said the first African-American First Lady has "literally" changed the face of the office.
With consistently high approval ratings, she's given a new generation of girls and women around the world a role model.
Describing Michelle as a "fashion icon and an athletic mother of two," Forbes said Michelle has "made the office of First Lady her own."
With 54 per cent Americans calling her a favourite, Obama is "a forceful advocate of school nutrition standards and military families' affairs."
The US First lady is more involved in policy than her predecessor Laura Bush was.
"But unlike Hillary Clinton, who championed a secretive (and ultimately unsuccessful) health care reform, Obama has stayed away from hard policy."
84-year-old Queen Elizabeth II is ranked 41 in the list. In the political category, the Queen is on the number 14 spot.
Forbes calls Michelle a "Jackie Kennedy" with a law degree from Harvard and street sense from Chicago's South Side.
"She's also effective: In response to her Let's Move! campaign against childhood obesity, companies like Coca-Cola, Kellogg and General Mills have pledged to reduce the calorie content of their foods by 2012."
Both Rosenfeld and Winfrey occupy the numero uno spots in their respective categories of business and media.
The lifestyle category is topped by 24-year old pop sensation Lady Gaga, who ranks 7th on the overall list.
Nooyi, 54, whose total annual compensation package last year was 10.6 million dollars, ranks second in the business category.
Forbes said Nooyi nudged a 20 million dollar slice of the company's USD 616-million-a-year ad budget away from traditional to social media spends.
Pepsi's worldwide campaign, Pepsi Refresh, allocates USD 1.3 million each month for a US project, such as the recent "Do Good For the Gulf," which offers stipends to build a shelter for animals whose owners lost their homes to the oil spill and to provide mental health services and job training.
"Brands have to speak to millenniums; young people want to make a difference," Nooyi says.
"We rely on these women for, yes, managing our money, creating paychecks and governing at home and on the international stage, but also what we eat, download, talk about around the dinner table and the causes we support," Forbes said.
Forbes said traditionally, women's lists are heavily salted with the language of "firsts." The firsts on the list include Nancy Pelosi, the first female Speaker of the House. She is ranked 11th.
Race car driver Danica Patrick (rank 93) is the first and only woman to have won an IndyCar series. Julia Gillard, the first female prime minister of Australia, occupies the 58th spot, while Johnson Sirleaf who is Africa's first female president, is 86th on the list.
Gillard is among 10 female heads of state on this year's rankings. There are 25 women on the list who hold CEO titles. In the business category, Axis Bank's Chief Shikha Sharma has been ranked 33, while ICICI's Chanda Kocchar comes in at the 35th spot.
Age wise, Forbes said "ripening for political leaders, generally, comes at the 50s and 60s, while business leaders peak in their 40s and 50s."
Media and lifestyle power women seem to come of age in their 20s and 30s.