Halle Berry's yowzah! from Elie Saab in 2001 is one of my favorites. Granted, I'm biased because it's my favorite color, but that top manages to be classy and outrageously sexy all at once. Few woman can pull this off and most would look cheap in the process.
Other memorable looks are memorable because they demonstrate a key fashion principle often forgotten in the scrum to stay off the worst dressed lists. At the 2007 Met Ball, Cameron Diaz remembered the importance of accessories and color. Keira Knightly used a big necklace to accent her dress for her first Oscar trip in 2006. (This dress reminds me of this painting, so it stuck in my head when a dress like that normally wouldn't.) Eva Green thought bigger for the the Cannes Film Festival and made a splash in Dior Couture. For her Oscar night, Julia Roberts went linear. Rihanna (in orange nails) and Marion Cotillard were textural sensations at the 2008 Grammys and 2009 Oscars respectively.
What are your favorite red carpet looks and why do you remember them?