Britney > Christina > Shakira > Beyonce > Avril > Pink > JLo
Edit: Reading through this thread and everyone dismissing Xtina's impact
Christina's voice alone has solidified her impact, she is seen to have one of the best voices of all time. Her voice still makes impact today, she is frequently being used on talent shows, being praised by legend and being viewed as an inspiration of the younger generation, even in her flop eras she is receiving honors. She is the Voice of Her Generation. People look at her recent failures and act like she isn't successful, her and Britney were the IT girls of the 2000's. the Dirrty video still getting referenced as the sexiest video of all time, Beautiful, Fighter, Lady Marmalade, etc.
This is just off the top of my head, obviously she has a lot more to her legacy but you all need to stop belittling her.