SUPERLATIVES

                            
The Magdalena is the longest                          Amazonas is the biggest department
    river in Colombia                                                           in Colombia

What is the longest river  in the world?

What was the most enjoyable holiday you have ever had?

Longest and the most enjoyable are superlative forms.

The superlative form is EST or MOST...  . In general, we use EST for short words and MOST ... for longer words.

Long -  longest               hot  -  hottest                   easy  -  easiest          hard  -   hardest

most famous                  most boring                      most difficult            most expensive

These adjectives are irregular:

good  --  best                        bad  --  worst                far  -- furthest

We normally use THE before a superlative (the longest / the most famous, etc...);

Yesterday was the hottest day of the year.

The film was really boring. It was the most boring film I have ever seen.

After superlatives we use IN with the places (towns, buildings, etc...)

Ex:  What is the longest river in the world?

We normally use OF for a period of time.

Ex: What was the happiest day of your life?

We often use the present perfect ( I have done ) after a superlative

That was the most delicious meal I have had for a long time

Taken from English Grammar in Use