Popular Music Question?

I got into a debate last night with a friend about the most popular band/rapper/singer in the world. He said it was Coldplay, however I wasn't convinced. I thought it was maybe U2, Red Hot Chili Peppers, or maybe even Lil Wayne. Does anybody know, based on record sales who is the most popular? If you could post a link that would be great to. Remember international not just the US. Thanks