Here are the best answers for the questions:

I. Choose the best answer:
1. Could you speak Vietnamese **more clearly** so I can understand what you mean?
2. That flight ticket is **the most expensive** of all.

II. Fill in the following blanks with a suitable form of words in comparison:
1. A holiday in the mountains is **more memorable** than a holiday in the sea.
2. My friend is **quieter** than me.
3. My garden is a lot **more colorful** than this park.

III. Underline the mistake and correct the whole sentence:
1. No mountain in the world is **bigger than** Everest. (Correct as is).

If you have more specific questions or need further assistance, let me know!