Well I'm sorry, but I have to go with Linda Blair... and I DON'T think I have to list the reasons why. [Although Heather Langenkamp WAS a close second!] But you know, I like Elvira, so it's all good. I'm glad she won, actually.
^ Oooh, I love some of the suggestions above: Anjelica Huston is classic and essential to my 90's nostalgia thanks especially to The Witches and the Addams Family films.
Glenn Close wasn't in many horror films but she gave great monster performances in the live-action 101 Dalmatians, Mary Reilly, Fatal Attraction, Dangerous Liaisons, and Cookie's Fortune. Only one of those is a horror film but she just kept doing roles where she screamed and was cold-blooded and psychopathic. She's awesome.
Adrienne Barbeau was in a George Romero film, a John Carpenter film, and a Wes Craven film- so she's kind of like horror royalty.
Elvira rules because she's the queen of Halloween and late nite horror iconography, in a way she wins this because she's the ultimate female icon.
I love Linnea Quigley and I hear she's done the most horror films, she also just wound up being in so many direct-to-video B-horror films I was planning to watch anyway.
Felissa Rose's performance in Sleepaway Camp is iconic for everyone who's felt out-of-touch with sex-crazied jackasses who think they're free and ... okay, this is starting to sound like a moral judgment but the sex-comedy kids and the stoners are infamous in my teen years as being people who were not the best to me. They weren't the outcasts, I was and they were not nice to me. So: I feel like this film proved this dynamic is actually fairly eternal and traditional. So, her role in this film and Felissa's performance are pretty key.
Jamie Lee was great because she was the thinking man/woman's sex symbol. After Halloween, all her horror films were about her portraying comparatively fearless, sex-positive characters who took no guff from people and were just healthy and well-adjusted women. She was an important figure for a reason.
Ricci and Ryder are also vital teen stars of the 90's and big parts of my childhood. Thanks to Heathers, Beetlejuice, Edward Scissorhands, and the Wednesday performances. (And a little bit of Casper.)
As for classic horror, I think special nods are in order to Carol Ohmart and Hazel Court. For me, personally, they had bigger roles than Barbara in Masque of the Red Death, House on Haunted Hill, and Spider Baby. But- they're all great.
And, who could forget Candace Hilligoss from Carnival of Souls? I couldn't.
Also... because I believe women-of-color need more representation in horror...
C.C.H. Pounder from Psycho IV: The Beginning is marvelous. She was also in Tales from the Crypt: Demon Knight, Sliver, The X-Files, as well as a bunch of stuff (TV projects mostly) that sound horror-inspired: House of Frankenstein, Ghost Cop, Things That Go Bump, W.I.T.C.H., and The Mortal Instruments: City of Bones.
Kelly Jo Minter is legendary for being a young black woman who did 4 noteworthy horror films: A Nightmare on Elm Street 5, The Lost Boys, The People Under the Stairs, and Popcorn.
Pam Grier deserves to be a legend for everything she's ever done, but she also to my knowledge has never wasted a chance to do a good horror film- Scream Blacula Scream and Bones.
Angela Bassett probably has more genre credits than I'm aware of but she was in Vampire in Brooklyn, Innocent Blood, and Critters 4. Only one of those is a good film, I'll leave it up to you to decide which.
...but I voted for Sigourney. Just because she's in my, like, top 5 favorite actresses. Period. And she played such a vital role in the 70's feminist film roles movement. And I'm a feminist. And that role also just happened to be in Alien, a horror film.
Here's a good reason why I may have chosen Sigourney: