What it means
の中で〇〇が一番 expresses that 〇〇 ranks highest within a stated group or range. It is commonly used with an adjective after 一番, such as おいしい or 好き, to say that something is the most delicious, the most liked, or the best in that group. The noun before の中で names the group, and the noun before が is the item being compared.
Usage note
一番 normally needs a following adjective or similar predicate to show what quality is being compared. The expression can be used in both familiar and polite sentences; the register is shown by the sentence ending, not by changing の中で or 一番.
How to use it
私の家族の中で母が一番元気です。
Among my family, my mother is the most energetic.
くだものの中でりんごが一番おいしいです。
Among fruits, apples are the most delicious.
私の家族の中で母が一番元気だ。
Among my family, my mother is the most energetic.
くだものの中でりんごが一番おいしい。
Among fruits, apples are the most delicious.