Правильно :
•who's the friendliest person in your class?
Nastay is the friendliest person in my class
• Is your best friend taller than you? (тут правильно)
No, he isn't. I'm taller than my best friend.
4. Which is the hardest lesson for you? (правильный вариант)
Maths is the hardest lesson at school for me.
5. Who has got the longest hair in your class? (правильный вариант)
Alina has got the longest hair in my class
6. Are you older than your best friend?
No, I'm not. My best friend is older than me или можешь написать I'm younger than my best friend.
7. Which is better - a picnic in a forest or a picnic at the seaside? (объяснение - nice=good)
A picnic in a forest is better than a picnic at the seaside.