I'd actually say Pink tbh....
She releases stuff that fits the musical sound of the time, but whenever she releases new stuff it all sounds the same to me.
Erm
Avril? Complicated, to My Happy Ending, to Girlfriend to What The Hell. Some gurls need to go and educate themselves and listen to her album's, each one sounds completely different.
The only moderately similar stuff Avril's released is the work she did with Dr Luke on The Best Damn Thing, and then the Max Martin tracks on Goodbye Lullaby.