the most / greatest … of all
Used to emphasize that something or someone is the most prominent or greatest of all in a group or category.
This pattern singles out one member of a category as the most outstanding. It is similar to saying 'the leading', 'the foremost', or 'the biggest' in English. For example, 日本きっての観光地 means 'Japan's top tourist spot'. It often appears with nouns like 名所, 企業, 選手, or 都市.
きっての is a formal and somewhat literary expression. You will encounter it in news articles, essays, and documentaries, but rarely in casual conversation. When speaking, you might instead say 一番 or 最も. Also, note that きっての must be followed by a noun; it cannot end a sentence.