adverb
Used to indicate that something is the most or best in a certain context. Often appears in phrases like 一の～ or 一に～. Somewhat literary or emphatic.
健康が一に大切だ。
Health is the most important thing.
彼はクラスで一の成績を取った。
He got the best grades in the class.
一番 is the common, everyday word for 'most' or 'number one'. 一 (いっち) is more literary and less frequently used.
Derived from the kanji 一 (one), used adverbially to mean 'first' or 'most'. The reading いっち is a variant of いち, possibly influenced by classical or dialectal usage.