Niagara Falls is famous as one of the world's leading tourist destinations.
Literal
Niagara Falls [topic-は] world-leading [possessive-の] tourist-destination [as-として] well-known is.
A descriptive sentence typical of travel guide writing. 世界有数 ('world-leading, one of the foremost in the world') is a common compound: 有数 literally 'having a number' means 'ranking among the few'. 名高い (なだかい) is a slightly literary i-adjective for 'famous, renowned' — uses the same 高い ('high') as in 'high standing'.