Where "best" = best developed, best written, most heroic (or villainous), most compelling (so factoring in the "she's insanely beautiful" portion of your vote goes here), intelligent/witty, and has the most impact on whatever plot threads make you love that show or movie.
This list mostly consists of characters from media considered "space opera," has space travel as a main feature, is or set on an alien planet.
http://www.ranker.com/list/tv-and-film-best-female-characters-sci-fi-film-and-television/admiralcrunch,
Beverly Crusher
Deanna Troi
Kara Thrace
Kathryn Janeway
Leia Organa
Seven of Nine
Talia Winters
Uhura
Caprica-Six
Gwen DeMarco/Lt. Tawny Madison
from Galaxy Quest