I'll start by saying that I think Jennifer Aniston is one of the most gorgeous women on the planet. However does anybody else think that the character is a b@tch.
She lends Ross on a merry dance more times than I can count throughout the series. When she's not with him she can't stand it when he meets someone else and when she is with him everything is his fault.
The first example that comes to mind is what she did to poor old Bonnie in the beach episode. She proceeded to come on to Ross, talk Bonnie into doing something she knew Ross would hate and then after she finally gets him to break up with someone he was happy with and get back with her. She then outlines the conditions of the new relationship.
Second example, she knows how it feels to see Ross with other people yet she proceeds to date (what's his face she used to work with) and she rubs that right in his face.
I can come up with more given time, but what a b@tch :-).