In terms of Billboard success, Katy is like Michael Jackson
Nobody now is like Madonna. Madonna was made in a time where superstars really, truly were superstars. "Superstars" are made in a China-based factory now, even Bieber and Gaga.
I think Rihanna is the artist sharing a similiar type of success as Madonna (the #1's, international success), she's also practicing the personality shifts (nice girl, bad girl, naughty girl, sex addict). Then there's the abusive "love of life."