4. "Dark Horse" (feat. Juicy J)
Released: December 17, 2013
Chart Peak: Number One (Four weeks)
Beware Katy's awesome power to overhaul the English language. This time last year, a "dark horse" was just an underdog. One smash hit later, that phrase instead conjures a malevolent witchy seductress that not even Juicy J can hit and quit. Dr. Luke's modification of trap music elements is no less drastic.