1. 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
1.1. EXAMPLE: She is (the) most beautiful.
1.2. This café is (the) best.
1.3. John and Lisa are (the) most intelligent.
1.4. This bowl is (the) biggest
2. This is not possible when the adjective comes directly before the noun
2.1. EXAMPLE: He is fastest swimmer.
3. 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'
3.1. EXAMPLE: She's the most beautiful girl I've ever seen.
3.2. It's the best café in London.
3.3. John and Lisa are the most intelligent students here.
3.4. This bowl is the biggest one.
4. Remember, we don't use 'the' when there is a possessive