Here are the completed sentences with similar meanings:

1. We have never stayed at a worse hotel than this.
➔ This is the worst hotel that we've ever stayed at.

2. I think Da Nang is not as exciting as Ho Chi Minh City.
➔ I think Ho Chi Minh City is more exciting than Da Nang.

3. There is no better teacher in this school than Mr John.
➔ Mr John is the best teacher in this school.

4. The Central Tower is the tallest building in this city.
➔ No building in this city is taller than the Central Tower.

5. Mount Everest is the highest mountain in the world.
➔ No mountain is higher than Mount Everest.

6. Real Madrid is not as good as Real Zaragoza.
➔ Real Zaragoza is better than Real Madrid.

7. He thinks no language in the world is more difficult than Chinese.
➔ He thinks Chinese is the most difficult language in the world.

8. He is the tallest boy in his class.
➔ No one in his class is taller than him.

9. She sings better than anybody else in the class.
➔ She is the best singer in the class.

10. Munnar is prettier than most other hill stations in India.
➔ Munnar is the prettiest of most other hill stations in India.

11. Mary thinks Melbourne is the most "liveable" city in Australia.
➔ Mary thinks Melbourne is livelier than any other city in Australia.

12. Lifestyle in Malaysia is the same as Thailand.
➔ Lifestyle in Malaysia is not different from that in Thailand.

13. Beijing is much busier than Ho Chi Minh City.
➔ Ho Chi Minh City isn't as busy as Beijing.