The silk gown took top spot in the poll by Sky Movies and In Style magazine, beating Marilyn Monroe's white dress from The Seven Year Itch and Audrey Hepburn's little black dress in Breakfast At Tiffany's.
Olivia Newton-John's tight leather trousers in Grease, Kate Winslet's blue gown from Titanic and Diane Keaton's tie and waistcoat in Annie Hall took the next three places.
The top ten was completed by Nicole Kidman's corset from Moulin Rouge, Liza Minnelli's fishnets and bowler hat in Cabaret, Cate Blanchett's gown in Elizabeth: The Golden Age and Vivien Leigh's lavish costumes in Gone With The Wind.
Sky Movies director Ian Lewis said: "There's no doubt that Keira Knightley's dress in Atonement created a massive stir this year, so it will be interesting to see whether it attains this iconic status in years to come."
> Click here for Digital Spy's guide to the best movies of 2007