greatest
adj.the best or most important of its kind. You use this to describe something that is better than everything else in a group.
adj.the superlative form of 'great', indicating the highest degree of quality, importance, or size. Often used to denote preeminence in a specific field or category.
“She is the greatest tennis player in history.”
“The museum houses the greatest collection of Renaissance art in the entire country.”
“Historians often debate which leader faced the greatest challenges during the transition to democracy, as each era presented unique social and economic pressures.”
Superlative form; almost always preceded by the definite article 'the'.
Greatest is already a superlative; adding 'most' creates a double superlative error.
Usage greatest challenges · greatest challenge