There have been many sex symbols in Hollywood - men and women alike - but nothing compares to the sex symbols of old Hollywood, back when stars were mysterious (instead of discussing everything in the press) and took part in both genders' dreams.
Women sex symbols were the ones every girl wanted to be - to mesmerize the male idols of the time, to have that certain something that makes everyone stare at them. There were especially two Hollywood ladies 'mortal' women aspired to be: Lauren Bacall and Marilyn Monroe.
Bacall, the femme fatale of film noir rose to fame after legendary filmmaker Howard Hawks gave her a role in To have and have not (with Humphrey Bogart): Bacall was 19 (but already had a Harper's Bazaar cover in her pocket). Bogart - whom she married in 1945 - was also her partner in Hawks' other movie, The Big Sleep, a famous and exciting example of film noir.
Bacall is the perfect example of the femme fatale - on and off-screen alike (her relationship with Bogart started back when he was still married; also married Howard Hawks fell for her as well, it caused problems between the two men). She was the woman whose innocent eyes drove the opposite sex crazy but she was also the woman who tried to do everything on her own - strong, beautiful and irresistible.
At 84 Bacall is still making films and though her golden years are gone, she's still wonderful.
Her How to marry a millionaire costar Marilyn Monroe was not that lucky - when she died at age 36 it was a shock to the world (it is still unknown whether it was suicide or homicide).
Her most famous film, Some like it hot with Tony Curtis and Jack Lemmon is widely considered the best movie ever (probably because it's impossibly funny) but her career - despite all the A-list films - is only one of the many things she is remembered for. Monroe is THE female icon of the 20th century, the woman no man would say no to if she happened to live nowadays and her spirit has not faded over the decades (not to mention the books and other media scrutinizing her life and mostly her relationship with the Kennedys and the brilliant Arthur Miller) and she is one of the most popular subjects of various merchandises - think the mugs, the clocks and others.
I think these women were born to be admired and no matter how many times they are copied by contemporary celebrities, they will always be the originals and you know, in 99% of the cases, original equals best.