1 He is the -most strong- boy in our class.
правильно:
He is the strongest boy in our class.
2 That cake is the -baddest-.
правильно:
That cake is the worst.
3 My book was -interestinger- than this book.
правильно:
My book was more interesting than this book.
4 He was the -handsomest- man in my family.
правильно:
He was the most handsome man in my family.
5 His flat was -biggest- than his friend's flat.
правильно:
His flat was bigger than his friend's flat.