8. Kristen vs. Lea
As seen during: Season Five
Having a BFF to share your deepest, darkest secrets with is great. We all need support and love and all that shit, so no shame. Only problem, however, is the fact that if you do have one person out there who knows your weaknesses, you better hope you never get into a fight with him or her. They'll know exactly where to hit you to ensure it hurts. We're nottalking figuratively, either.
When Kristen punches Lea, thus commencing their BFF smackdown, she clearly knew exactly what button to press (or, rather, what side of her jaw her fist should collide into). The first strategy of war, of course, is to know your enemy. And no one knows a bad girl better than her own BFF.