Ответ:

  1. But it's got the biggest screen.
  2. It's the prettiest case in the shop!
  3. I'm the hungriest person in the world!
  4. My mum makes the best cakes!

Сравнительная степень заменена на превосходную: big → the biggest, pretty → the prettiest, hungry → the hungriest, good → the best.