Which sentence states a fact?

 - Mia Hamm was the most impressive soccer player in the history of the United States Women's National Team.
 - Mia Hamm helped her soccer team at the University of North Carolina at Chapel Hill win four NCAA titles.
The second sentence states a fact.
Mia Hamm helped her soccer team at the University of North Carolina at Chapel Hill win four NCAA titles.
It can be proved by reading a biography of Mia Hamm.
The first sentence states an opinion.
Mia Hamm was the most impressive soccer player in the history of the United States Women's National Team.
Most impressive shows what a person believes, thinks, or feels. Another person might have a different opinion about which player in the team's history is the most impressive.