The nicest waterfall I know in NYS is Kaaterskill falls in Ulster County in the Catskills. It’s a two tiered waterfall with the upper falls 175 feet and the lower falls 85 feet. It’s at the end of a short (maybe 1/4 mile) trail just off state highway 23A. It’s very pretty, not too heavily visited (go one a weekday and you should have it to yourself), and has some good swimming holes in the river below the falls.
The World of Waterfalls has been to this waterfall, which you can read about here.