Dude, you do not want to mess with Regina George. Even a bus couldn't take her out! With her Army of Skans at her side, she rules her school with an iron fist and a smile on her face.
Here's to you, Mrs. Robinson. It should be all about Ben and Elaine, but she's the one who really owns the movie as evidenced by the fact that the character became a cultural template, the name shorthand for relationships between older women and younger men.
Most of Bette Davis' characters could have ended up on this list, but Margo is the most memorable and perhaps the most quotable. She is arguably also the only one on this list who really comes out on top in the end, perhaps because her misdeeds are less grave than those of the others. Still, if she were to say "jump," I wouldn't hesitate if I were you.
Evil to the core, a master manipulator who manages to play everyone even, as it turns out, herself. I'd suggest staying on her good side but she doesn't really have one as friend/rival/enemy Valmont could attest.
Just stay out of Scarlett's way. If a job's gotta get done, she'll take care of it, but she might just smack you down in the process. Still, in a time of crisis, she's the one I'd want to be around because few people take care of business as effectively as the divine Ms. O'Hara.
* That's Head Bitch In Charge, in case you're wondering