Neither!
Gaga & Rihanna are their own artist, who have made an astounding amount of impact on Pop Culture in their own right. Their impact doesn't need to be compared to Madonna or vice versa.
Both of them are in the process of creating their own legacy and their impact is still on going and is very powerful.
I personally don't think it's a good thing to be compared to other artists, especially legends, in any which way. It isn't fair to both parties due to the generational gap, different scenarios & circumstances. Every artist wants to be their own individual presence!
