Madonna

Just when people thought they understood Madonna, she'd change her hair, sound, clothes, attitude and probably start another argument.

That restlessness is the whole case. Madonna didn't survive pop by staying comfortable. She kept finding the next version of herself while everyone else tried to catch up. The songs mattered — a lot — but so did the nerve. Sex, religion, fashion, videos, controversy: she understood that being a pop star meant controlling the conversation beyond the speakers.

Michael Jackson had bigger spectacle. Beyoncé brings a different level of precision. Madonna's weapon is reinvention. She made longevity feel confrontational.

So how much should changing pop's rules count? If owning one era is impressive, what do you call somebody who kept coming back to claim another?