Utah again once they leave for the Big Ten. UCLA they will still think about; Notre Dame they will. Even Washington will be more at the forefront of USC's mind than Utah.
Not throwing shade at Utah; they've been the best team in the Pac 12 South for 5 years or so now. But that doesn't make it a rivalry, it just means that USC couldn't win the South without beating Utah (and they didn't win the South for a long time due to a knee jerk, emotional/stupid HC hire...).