Hits They've Written: Taylor Swift's "We Are Never Ever Getting Back Together," Taylor Swift's "I Knew You Were Trouble," Ariana Grande f/ Iggy Azalea's "Problem"

A protege of Swedish legend Max Martin, Shellback is one of the most in-demand songwriters and producers in music today. He co-wrote and produced singles "We Are Never Ever Getting Back Together" and "I Knew You Were Trouble" for Taylor Swift's Red album in 2012, and this year has put in work co-producing Ariana Grande's "Problems." He's also worked with Icona Pop on their debut LP, as well as the recently-released Lily Allen LP, Sheezus.