Any female that achieves a great deal of success is going to be compared to Madonna (Beyonce, Rihanna, Gaga come to mind too), and it showcases how much disrespect the mainstream has towards Madonna's legacy. It seems like they've just devolved her career achievements into "successful person with vagina," as if any of the above women have done anything remotely aligned with Madonna's career shifts, let alone come CLOSE to replicating Madonna's genius and sustained success.
Madonna is her own beast, and no one has EVER or WILL ever come close to playing the game like she has. Britney's self-inflicted career suicide like... 5 years after her debut is enough to suggest that she's not in Madonna's league. Beyonce is the only one (so far) who has sustained a successful career for over a decade, but she's done so with safer career choices, and hardly any visual transformations. She's safe and likable enough to stay on mainstream's good side all these years, and she's held on to her winnings the whole way through to become the juggernaut she is now. Rihanna shares Madonna's old affinity for self-transformation, though Rihanna's never come close to capturing the zeitgeist like Madonna has done time and time again. I suppose Gaga's taken the same "hard choices" that Madonna's taken but not really in a challenging, provocative way that's pissed people off... but more like adding to what she's done before and annoying the public with the excess. Very different reaction. We'll see how Gaga builds and sustains her career before comparing her to Madonna, but until then, there will only be one. And nothing's more disrespectful to Madonna than having the nerve to suggest that Britney Fucking Spears is at her caliber.
Musically? She's always a few months ahead of the curve. She and her team have a lock on the pulse of youth culture and it keeps her relevant album after album after album. Even early in her career, something like SOS was not the current sound, it was from the future. It's very Madonna. Beyoncé can be super-current when she wants to, but never game-changing sonically. That's the Rihanna difference.
Janet was about 3 years ahead with Control. Your fave could never.