～は～の一つだ【は～のひとつだ】

[A] は [B] の一つだ means "A is one of B": A belongs to the group B. B is often a noun with a description in front of it – "famous mountains", "my favourite films" – and the sentence ends in ひとつ一つです in polite speech.

Japanese has no plural here: B stays as it is, where English says "mountains", "films".

一人 for people

When the group is made of people, 一つ becomes ひとり一人.

• × 彼は選手の一つです → 彼は選手の一人です

With 一番

"One of the most…" puts いちばん一番 before the adjective, and the range – "in the world", "this year" – goes before it with で or のなか中で.