We use a superlative to say that a thing or person is the most of a group. When we use a superlative adjective ('the tallest student') before the noun, we generally use it with 'the'. This is because there's only one (or one group) of the thing we are talking about. There is one student who is the tallest in the class, and because it's clear to the listener which one we mean, we usually use 'the':
-She's the most beautiful girl I've ever seen.
-It's the best café in London.
-John and Lisa are the most intelligent students here.
-This bowl is the biggest one.
Remember, we don't use 'the' when there is a possessive:
-He's my best student.
-That's our most important goal.
It's possible to drop 'the' when the adjective is used later in the sentence, rather than directly before the noun. We can choose either 'the' or 'no article', with no change in meaning:
-She is (the) most beautiful.
-This café is (the) best.
-John and Lisa are (the) most intelligent.
-This bowl is (the) biggest.
-This is not possible when the adjective comes directly before the noun:
He is fastest swimmer.
With superlative adverbs, we can also choose to use 'the' or 'no article'.
-Luke reads (the) fastest.
I like vanilla ice cream (the) best.
-She can speak six languages, but she speaks Spanish (the) most confidently.
These examples all compare one person or thing with other people or things. However, sometimes we compare a person or thing in one situation with the same person or thing in a different situation. In this case, when the superlative adjective or adverb is later in the sentence, we usually don't use 'the'. Compare these two sentences:
-I'm most productive early in the morning [I'm more productive in the morning than I am in the afternoon or the evening].
-I'm the most productive early in the morning [I'm more productive than the other people in my office first thing in the morning].
More examples:
-Julie does swimming, running and cycling. She's always most tired after cycling.
-John types most quickly when he's drunk a lot of coffee!
-Tea is˜ best when you drink it very hot.
-London is most depressing in January.