Narrowing things down in this way makes this list fairly difficult to create. Unfortunately, there aren't a lot of great female leads in science fiction, something I hope will be remedied in the coming years (girls are awesome too, you know). So, without further delay, here are my top ten leading ladies in science fiction:
Ellen Ripley / Sigourney Weaver (The Alien Series)
What list could be complete without Ripley? She is the quintessential science fiction leading lady. If you don't believe me, then you need to see the ending for Aliens. Taking on the Queen Alien in a mechanized loading vehicle is pretty ballsy. But I'm sure the Queen got what she deserved.
Sarah Connor / Linda Hamilton (The Terminator)
She becomes a stronger character, physically, later in the series, but her first appearance is a memorable one. After all, she falls in love with a man who technically doesn't exist yet and manages to defeat the Terminator in the nick of time! And don't forget that her character grows into a bit of a badass later on.
Evey / Natalie Portman (V For Vendetta)
I think Evey's psychological progression in the movie (and the comic) is a powerful and relatively unique portrayal in science fiction. It's not often that we see movies that are well aware of the mental states of the characters. V For Vendetta does a fine job turning Evey into a complex female figure worth remembering.
Captain Kathryn Janeway / Kate Mulgrew (Star Trek: Voyager)
I've always been a fan of Voyager. My love for this Star Trek derivation comes from its unique characters. From Seven of Nine to Captain Janeway, Voyager is one of the few Star Trek series that, I think, really tries to push the envelope. Maybe I'm wrong, but Janeway is one of the top captains in Star Trek, only rivaled, in my opinion by the two biggest names in Star Trek (Picard and Kirk). It would be interesting to see Janeway make an appearance in a movie somewhere, because her character deserves more than just a television show.
Laura Roslin / Mary McDonnell (Battlestar Galactica)
You'd have to be a fan of BSG to know why Roslin is on this list. For a character who became President by a string of seemingly random deaths, Roslin is pretty much the ultimate of badasses. She's not afraid to chuck someone out of an airlock for pissing her off and she tends to get what she wants, even if nobody else wants it. That, and Mary McDonnell managed to turn this character into one of the most astonishingly complex figures in science fiction history. More so than any other. But that's BSG for you: a show that is about characters, that knows it's a good show and isn't afraid to show it.
And that's that. Unfortunately I couldn't think of any good female figures in literature, so it seems this list will be primarily about film and television. But if you know any awesome female leads in science fiction literature, let me know in the comments. The same goes for anyone I might have missed that deserves to be here!