Taylor Swift has earned the No. 1 spot on Billboard's Top Artist of the 21st Century chart based on her stellar performance.

John Lennon didn't care about Paul McCartney's solo songs but he liked a Wings hit that sounds like it was performed by Kermit the Frog.

But the reality is often subtler than that—career-wise, each superstar needed something from the other. Often, one act is a ...

But the reality is often subtler than that—career-wise, each superstar needed something from the other. Often, one act is a ...