Rihanna, back to defend her title, has better instincts for what listeners crave in the hot months: mordant ballads about love and light dance tunes about lust. Take a Bow, her ballad, is unremarkable, but Disturbia has one of those rubber-ball melodies that bounce around your head even before Rihanna hammers it home as a lyrical refrain ("Bum bum be-dum, bum bum be-dum bum"). Co-written by R&B star Chris Brown, Disturbia ingratiates itself by not trying to be more than the silly piece of product that it is. But a summer anthem should have at least one moment when it feels enormous.