Correção de frase "These exercises are the most difficult"

Hi;
Teachers, arrisquei nessas frases, poderiam corrigir?
Obrigada!

These exercises are the most difficult in the whole book.
This summer is hottest than last summer.
This chair is the most confortable chair in the whole house.
John is much more clever than his brother.
Mt. Everest is the most high mountain in the word.
The Pacific is the largest ocean in the world.
Which is the best route from Miami to N.Y..
This street even wider than Broadway.