This was tough. I might as well have said this was in no particular order, because I keep changing my mind.
10.) Seven of Nine (Star Trek: Voyager)
I do think Seven got overused in the series. But I think Jeri Ryan is cute (still is) and I have to admit she's hot in both full Borg mode and humanoid mode. Especially when the doctor asked her to take her hair down.
9.) Bellatrix Lestrange (Harry Potter)
This is strange but I didn't find Helena Bonham Carter hot until she played Bellatrix.
8.) The Borg Queen (Star Trek: First Contact/Voyager)
*sappy sigh* Oh how I love Alice Krige. And I love monstrous humanoids...
7.) Madame Vastra (Doctor Who)
Like I said, I love a monster/demi-human. Crime-fighting, literally man-eating, sapphic reptile. Fortunately Steven Moffat says we'll see more of Vastra (and Jenny, yay!).
6.) Chiana (Farscape)
I love her, I hate her. And still so cute. Damn it, Pip.
5.) Leeloo (The 5th Element)
My first very welcome exposure to Milla Jovovich.
4.) Julie Walker (Return of the Living Dead 3)
I love Mindy Clarke. This film is a real laugh, but it's got some great makeup, inclluding on Julie. Hottest zombie ever? I think so.
3.) River Song (Doctor Who)
River would have been higher on the list, but lately all this retconning and Mary-Sue-ing of her has annoyed me. Oh well... still awesome. Just not as much as she used to be.
2.) O-Ren Ishii (Kill Bill Vol. 1)
My favorite viper. Ice-cold badass bitchiness to perfection.
1.) Catwoman (Batman Returns)
I have a thing for Catwoman, can you tell?