Learn Italian: Questi volontari sono le persone migliori che conosco. - These volunteers are the best people that I know.

In Italian, to form the superlative ("the best [noun]"), you use the definite article before the noun, but you don't repeat it before the adjective. So "the best people" is just le persone migliori, not le persone le migliori.

Wait, migliori ends in -i, but persone is feminine plural ending in -e. Why don't their endings match?

The adjective migliore (better/best) ends in -e in the singular. Adjectives that end in -e in the singular change to -i for the plural, for both masculine and feminine nouns. So it is la persona migliore (singular) and le persone migliori (plural).

Why use che for "that/who" instead of chi?

Che is the standard relative pronoun used to connect a clause to a noun that was just mentioned (here, le persone). It translates to "who", "whom", "that", or "which". Chi is only used to mean "he who" or "the people who" when there isn't a specific noun right before it, or in questions.

Why is it Questi if the sentence later talks about le persone, which is feminine?