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The Duchess of Cambridge's mix of high and low looks - designer fashion one day, jeans the next - and a "whirlwind year of fashion successes" earned her a spot on the 72-year-old list, according to a statement from the magazine. She had appeared once before, in 2008.
Other royals who caught the eye of the designers, retailers, editors, socialites, photographers and entertainers who submit ballots include Princess Alexandra of Greece, Princess Charlene of Monaco, Italian Countess Matilde Borromeo and Sheikha Moza Bint Nasser of Qatar.
Mrs Obama, a four-year veteran of the list, was named for the first time as part of a best-dressed couple, although President Barack Obama had been named on his own to the list once in 2009. Bruni-Sarkozy is another repeat recipient.
First-timers include Justin Timberlake, Colin Firth, Armie Hammer and Christine Lagarde, chief of the International Monetary Fund. Carey Mulligan and Tilda Swinton have each made it twice. News anchor Brian Williams was named to the hall of fame.
Fashion professionals get their own category on the list; model Stella Tennant, photographer Mario Testino, Burberry designer Christopher Bailey and couple Lauren Bush and David Lauren were among those to make the cut.
Lady Gaga, Janelle Monae, the King of Bhutan and couple Vanessa Paradis and Johnny Depp fill the "fashion originals" group.
Vanity Fair inherited the list, considered one of the most influential in the fashion world, from the late fashion publicist Eleanor Lambert in 2004.