The most popular song right now is Somebody That I Used To Know. It was done by Belgian-Australian singer and songwriter Gotye featuring Kimbra. The song is from his third album Making Mirrors. It was released on 6 July 2011 as the lead single from the album.
if you ever want to know what's the most popular in ANY genre, Billboard will tell you, but for radio it's currently We Are Young by a band called "fun", they obviously don't believe in capitalizing their name.