I realize "best of all time" is an impossible list, but I think this dozen (or so) will stay iconic.
Neither of these outfits are my favorite Diana outfits but they are both iconic. The baby blue dress was inspired by Grace Kelly's outfit in To Catch a Thief and was later auctioned off for charity. This was likely a call back to her friend who met her husband, the Prince of Monaco, when she was in the French Riviera for the Cannes Festival. Catherine Walker's fashion label is still popular (website).
There are photos of Elizabeth Taylor wearing her tiara to a previous Cannes festival but I cannot verify if they are real or not. Before the 90s, re carpet photos of Cannes were not as popular. It was hard to obtain good pictures or verify the location. In 1987, Elizabeth walked the red carpet in a stunning red dress whose silhouette defined the 80s (and bridesmaid dresses for the ages). Nolan Miller famously designed dresses for Dynasty and Charlie's Angels. His main business was for TV, but he did have a QVC line with Joan Collins. A few dresses can be found in vintage stores (website).
Shortly after McQueen's death, Cate Blanchett walked the Cannes runway in one of his creations. His fashion line still continues (website).
Naomi Campbell's gold dress fits her impeccably! The Italian designer Roberto Cavalli's fashion label is still going, but there was rumors of bankruptcy in 2019 (website).
Fan Bingbing wears a wide range of clothes to Cannes, pushing the boundaries of costume, traditional, and couture. Here, she stunned in a red dress made by an up and coming designer. I cannot find any articles of the designer outside mention of this red dress.
Laetitia Casta was eye-catching in a gold feather cape in 2013 by wearing an Erik Halley cape and Christian Dior gown at the Cannes Film Festival. Laetitia Casta is a French actress but she is more commonly known in the USA as a Victoria's Secret model. Sadly, it appears Erik Halley only does commission pieces and does not regularly create fashion garments. Dior is a still active French fashion house (website).
Blake Lively's Gucci dress is a classic (website).
This Vuitton sequin dress fits Léa Seydoux like a glove and I love it. The sliver to Naomi's gold. Louis Vuitton is still popular (website).
Bella Hadid's lung dress (mentioned in my honorable mentions) is likely more know, but I personally adore this quilted look and I think it was one of the first of this 2010-2020s trend.
Channeling mid-1900s classic Dior, Elle Fanning stunned in 2019 by wearing a timeless Dior dress at the Cannes Film Festival.
Lori Harvey, a model and daughter of Steve Harvey, stunned in a yellow gown shortly after her breakup with Michael B. Jordan. Alexandre Vauthier is a modern French couturier (website) and Messika is a modern French jeweler (website).
This is likely my most controversial pick as black and gold stripes are rare on the American runway. However, I adore this combo in a sparkly sari and I think it's Deepika Padukone's best look. Sabyasachi Mukherjee is a famous Indian fashion designer (website).
To make this is a baker's dozen here is my "honorable mentions"
Catherine Deneuve, 1966
Vanessa Redgrave, 1967
Diane Kruger in Vivienne Westwood, 2012
Jessica Chastain in Alexander McQueen, 2016
Bella Hadid in Schiaparelli, 2021
Gemmas Chan in Oscar de la Renta, 2021
