There are a lot of female characters in books that I really like, but I can't say I absolutely loved and am actually haunted by any of them besides Rebecca. I love many, to be sure. Elizabeth Bennet, Yelena (Poison Study), Lady Emily (Lady Emily mysteries), Lady Julia Gray, Bess Crawford, Rose & Sydney from Vampire Academy...etc, but Rebecca holds such a mystique.
She has such a presence in the book, without ever being in it. She haunts the lives of those she left behind. Is she a wicked woman? Is she a good woman, whose reputation is tarnished by the bitter memories of a man who couldn't keep her under control? By a woman haunted by her husband's memories of the woman he married but could not possess? We don't know.
There have been "sequels" to the original that have been nothing more than poorly written and imagined fanfics, that have tried to give a voice to her memory, and I've read almost every single one of them, however bad because I am fascinated by her, still.