Of all non-Broadway recording artists, Madonna has always been my favorite. I think she has just lead a legacy that we can see if others will follow, but I highly doubt they will come close.
And as for the Britney/Madonna comparisons...here's what I personally think is the one difference--Madonna actually had vocal talent! If you listen to the Immaculate collection, she has a pretty voice and just hits everything. Listening to most Britney songs makes me want to vomit, in my opinion, you just cannot compare the two vocally.
"If there was a Mount Rushmore for Broadway scores, "West Side Story" would be front and center. It snaps, it crackles it pops! It surges with a roar, its energy and sheer life undiminished by the years" - NYPost reviewer Elisabeth Vincentelli