I think Justin definitely had an impact on Pop Culture, whether it was his relationship with Britney, his ''look'' (shaved head, suits) and in particular his second album FutureSex/LoveSounds because it's had a massive influence on the Pop'n'B dominating the charts today. He is an Icon.
Usher had the chance, producing a fantastic album like Confessions, but then he blew it. He's now almost completely irrelevent because he's having no influence on Music and just following the crowd.
Chris Brown also threw his chance away when she beat Rihanna. He's still successful, but the potential and momentum he had before that incident was extraordinary. He could have been great, now he's not.
Camp Male Pop Artists like Mika and Adam Lambert have been successful but are underperforming.
Rappers like Jay-Z and in particular Kanye are certainly having an impact on Pop Culture, but they're not Pop in the context we're discussing.
And then we have Singer/Songwriters and bands, who can be very successful but again, aren't the kind of Pop we're discussing.
It really is female domination.

My Gawd, it's Blond Ambition.

Madonna's a friggin' genuis.
I blame her.