The 36-year-old actress, who graced the magazine's August issue cover, shared the honour with new mother Kate Middleton.
Washington has worn stylish dresses in various red carpet appearances. She donned a Miu Miu dress at the Oscars and looked stunning in a lemon yellow Jason Wu gown at CFDA Awards.
Middleton makes it into the list for the fourth time. The magazine noted that her notable outfit for 2013 is a pink Alexander McQueen coat and Jane Corbett hat which she wore at the Trooping the Colour ceremony in June.
Beyonce Knowles, Charlize Theron and Brooke Shields are among celebrities who make debut appearance on the best-dressed list.