So many of my favorites have already been named, but as a movie buff I have plenty to add here.
I'll start with What A Way To Go! starring Shirley MacLaine. It's loaded with over the top costumes by Edith Head. Here's a sample:
(credit: DVD screencaps sent by a friend, ImageShack for hosting)
Some of my other favorites that are already on this thread:
Sharon Stone in Casino, posts 365, 366, 368 (my favorite)
Kim Novak in Vertigo, posts 331, 332, 333, 334
Tippi Hedren in Marnie, posts 322 & 323
Diana Ross in Mahogany, post 218
Natalie Portman in the Star Wars series posts 152 & 441
Elizabeth Taylor in Cleopatra, post 321 and Cat On A Hot Tin Roof, post 168
Madonna in Swept Away, post 169