best in the world; number one in the world
Used both as a noun and adverbially to describe something as the best or top-ranked globally. Often appears in phrases like 世界一の～ (the world's best ~) or 世界一～ (the most ~ in the world).
I think Mt. Fuji is the most beautiful mountain in the world.
彼は世界一のピアニストを目指している。
He aims to become the world's best pianist.
This company's technology is the best in the world.
He is the best pianist in the world.
Everest is the tallest mountain in the world.
She keeps training with the goal of becoming the best in the world.