When we want to say that something is the best, we use –est. We usually use the before these words.
When the word has only three letter (consonant + vowel + consonant), we double the last letter.
the simplest / the most simple
the happiest / the most happy
When the word has two syllables, we can use –est or we can use most. When the word has two syllables and it ends with –y, we change –y into –iest.
the most interesting
the most important
When the word has three syllables, we use most.
good – the best
bad – the worst
Some words use special forms.
You can see more examples in the book Robinson Crusoe.