Since today is Sigourney Weaver’s birthday, I think it’s probably a given that today’s scene of the day would feature her defeating an alien.  In this scene from 1979’s Alien, Ripley shows why she is the last human survivor of the Nostromo.

(As cool as Ripley is, she’s still nowhere close to being as much of a badass as Jonesy the Cat.  Jonesy just had to hiss and the alien knew better than to mess with the ship’s cat.)