This is the 2154th most frequent Japanese word.
日本一
"日本一" means "best in Japan" or "number one in Japan."
Here, 日本一 is used to describe 'the most in Japan' in terms of beauty, implying that there is no mountain in Japan more beautiful than Mount Fuji.
富士山は日本一美しい山です。
Mount Fuji is the most beautiful mountain in Japan.
In this sentence, 日本一 emphasizes being the best or number one in Japan, specifically regarding sushi chef skills.
彼は日本一の寿司職人だと認められています。
He is recognized as the best sushi chef in Japan.
The word 日本一 here is utilized to state 'the tallest in Japan', emphasizing a measurable or factual attribute.