Forming the superlative of adjectives in complex cases

E.g. Marie est la fille la plus intelligente de la classe (Marie is the smartest girl in the class)

Two things to pay attention to in complex examples like this are:

1. la fille usually appears before la plus whereas in English it's the reverse order. Also see that 'la' appears twice.
2. de la/du/des/ de l' ('of the') always follows the superlative, even when in English we might say 'in'.

Note: you can also say:

Marie est la plus intelligente fille de la classe (Marie is the smartest girl in the class)

See also the related Le, la, les plus and le, la, les moins = the most and the least (superlatives of adjectives)

Here are other Superlative structures:
Le plus and le moins = the most and the least (superlative of adverbs)
Meilleur, mieux, pire / plus mauvais, plus mal = better, best, worse and worst (irregular comparatives and superlatives)

And Comparative structures:
Plus... plus..., moins... moins... = the more...the more..., the less...the less... (comparisons with phrases)
Better and better, worse and worse = de mieux en mieux, de pire en pire (comparisons)
De plus en plus and de moins en moins = more and more and less and less (comparisons with adjectives, adverbs, verbs)
De plus en plus de and de moins en moins de = more and more and less and less (comparisons of nouns)
Making comparisons with adjectives: plus... que, aussi... que, moins... que
Making comparisons with adverbs: plus... que, aussi... que, moins... que
Making comparisons with verbs: plus que, autant que, moins que
Making comparisons with nouns: plus de... que, moins de... que, autant de... que