I imagine it would be hard to beat the Equinox Hotel, given that it was a high-end gym chain before they opened a luxury hotel. It's VERY expensive and not conveniently located unless you have a reason to be in Hudson Yards, but I don't think many (any?) other hotels in NYC have a 60,000sqft fitness center with indoor and outdoor swimming pools.

If that's overkill, I would look for smaller hotels that maybe don't have their own gym but offer guests a pass to a nearby full-service gym. Some googling suggests the Archer (New York Sports Club), High Line Hotel (Chelsea Piers), Civilian Hotel (TMPL Health Club), and Distrikt (Mid-City Gym) all fit in that category.