Gaga, because she, more-so than any other artist, defines what music videos, fame, and pop culture in general will look like for the next 10 years, something like what Britney did for the 00's.
Oh, and she does so while writing her own music the whole way.
Since Rihanna debuted onto the scene, she's accumulated more notable hits than any other artist period, so although she isn't the pop culture icon that Gaga is, her discography will be notable in 20 years as well
Katy lacks respect in the industry, but if she continues with the same success she's had the past year, she may end up like-wise with Rihanna. We'll have to see though.
