Courtney from Jawbreaker is a far better mean girl than Regina George
Seriously, she is truly a villain and her plans are more "sinister" and she actually gains the upper hand until by cheer luck they come across that recording at the right moment.
At no point during the movie does Courtney's position as Queen Bee gets undermined like it happens with Regina and whenever a threat pops up Courtney squashes it right away (like she did with the girl they gave a make over).
Courtney is underrated. She is THE mean girl.