Gaga's, because she really shaken up the whole pop industry and had amazing and huge set of singles and was one of the people who helped electro-pop to blow up. Her latest album did great too despite only having one proper and huge WW smash, but her tour made up for it.
Rihanna's path is different, and consistency is where she excells. It may or may not help her outpeak Gaga's overall success when it's all said and done, but she doesn't deserve to be discredited like she is by fan bases who usually try it with her.
