This is the best water I’ve ever drink.

 I’ve never had better water than this one.

2.     Lan’s hair is as long as my hair.

 Lan’s hair is the same length as my hair.

3.     Hoa is the most beautiful person in my class.

 Hoa is more beautiful than everyone in my class. 

4.     Do you have a ruler smaller than this ruler?

 Is this ruler the biggest you have ?

5.     The white car is cheaper than the black car.

 The black car is expensive than the white car.

6.     There is no one better than my father.

 My father is the best.