Co-written by Max Martin and Dr. Luke, this buoyant electro-pop singalong is 2010's catchiest tune. As for that "teenage dream," Perry doesn't mince words: "Let's go all the way tonight."
• Gallery: Katy Perry's Best Looks
Random Notes: Hottest Rock Pictures
Video: 'Captain America' Sequel Is Super
Gallery: Coachella 2014: Rolling Stone's Best Live and Backstage Photos