In fact, she was so good at being "just" Ripley that it wasn't until I saw Sigourney Weaver in other movies did I realize, "Huh, Sigourney Weaver is a woman and she is hot"...She is the only female character I can think of in which her gender has no real bearing to how she acts, what she says, or how she is presented on screen.
I challenge anyone to think of any other woman character who follows Ripley's mold:
1. Does not overtly seek a romantic relationship
2. Sexual tension is not a compelling foil or conflict for the character
3. Is not thrown into an action role purely for the spectacle of a woman kicking ass
4. Is not shouted down or treated differently specifically because she is a woman.
5. Is not shown in a gratuitously sexual light (she is in her underwear the same time everyone else is logically in their underwear).
6. Is unequivocally the hero of the movie. In fact, in both movies [spoiler alert for 20 year old movies], it's quite notable that she is literally the last human being standing, and she wins on her own terms.
No other contenders, that I can think of, meet this standard, especially in the action film role: Catwoman, Aeon Flux, Tomb Raider...nope. Clarice Starling...even if you want to argue that no sexual tension exists between her and Hannibal Lecter (even though he specifically refers to it), much of the non-gruesome psychological conflict her character faces is precisely because she is a woman in a (tall) man's world. Beatrice in Kill Bill...great role, but again, it only works because she is a woman and it is a spectacle, even if empowering (i.e. could you imagine Kill Bill being made with a man in the role?)
The only woman character who comes close to the unisex appeal of Ripley is Marge Gunderson in Fargo. Other than that, it's pretty slim pickings...whereas for males, there are too many to count.
Anyway, I sometimes wonder how much growing up with Alien/Aliens affected how I think about gender roles and imbalance. Those being among the first and the most interesting of the action movies I'd ever seen (then, and today), I kind of think that the idea of a woman action hero was completely normal to me. And yet, as far as I can see, no one like Ripley has since been written...