I can't speak for all women, especially since I think my recommendations would be a bit odd. I tend to like stories that are not gory or horror filled and end with an uplifting message or redemption. I hate the over sexualized books which are more like soft porn and completely unrealistic (real people can't have sex without consequences). Finally, I studied science so when they get into the technical details I tend to be pulled out of the story and start analyzing and getting frustrated at the obvious errors or silliness of making it sound scientific by using science words but actually saying nothing.
I hate having horrible books imbedded into my memory (like
The Grapes of Wrath or the opening of a JD Robb book which described a rape scene after which I couldn't read anymore), thus I try to only read books where I have some assurance of it's quality and that it will end well.
So (sigh - I'm so going to get flack for this), I really liked
A Princess of Mars by Burroughs and a bunch of his other books. I think his earlier books are more fantasy, but later he drifts into scientific-fiction. They are all horribly written and generally the same theme: Guy meets Girl and falls madly in love, Girl gets kidnapped by evil mastermind, Guy meets new race/finds new technology, Guy rescues Girl with awesome fighting skills. The women in his novels are helpless damsels in distress who do just about nothing to help themselves, so I fear most modern women who abhor the idea of a man being able to do something they can't would hate these books. But I found it refreshing to read a book about a strong male character who was honorable and brave.
And they were free...
Actually, the more I think about it almost all "sci-fi" books I liked were more in the fantasy realm.
- Dune was probably the closest although I thought the second book odd and put it down before finishing.
- CS Lewis's space trilogy was fun, but more of a theological speculation.
- Girl Genius comic has been fun to read - although it's not a book but a steampunk comic
- The Silkie by Vogt was fascinating with how the silkie used logic as a weapon
- Does Hitchhiker's Guide count?