I always found those women with the four eyes and two mouths from Battlestar Galactica to be really damn cool. Not the most attractive, but that’s an image that stuck with me since I was a wee child.
Does Wonder Woman count? It’s a good toss in my mind between her and Xena (not sci, but defninitely fi).
If your definition of sci/fi includes the superhero genre, then my top choice would definitely be Lynda Carter as Wonder Woman! Other strong contenders would be Yvonne Craig as Batgirl; Caroline Munro as Stella Starr in the awful movie “Starcrash”; Michele Lintel and Joan Severance as Black Scorpion (Lintel starred in the TV version which followed the two Severance movies); Jeri Ryan as Seven of Nine; and of course Jane Fonda as Barbarella!
Sara Pezzini gets a big second. (If she’d only stop getting drunk and beating up her father, who used to sing with the Lovin’ Spoonful.)
Sigourney Weaver. Yes.
Definitely Kaylee. And River.
Barbarella. Ain’t no one took off a space suit like Jane Fonda.
I have to call a tie between Ripley (Weaver) and Sam Carter (Tapping). Both of those ladies can kick some serious ass. What I love about both characters is that they are simply women, not gratuitous eye candy (although I don’t usually complain about GEC), that are capable, strong, smart, and attractive but don’t dress in patches of spandex and leather.