There are a few places considered to be the best spots within NYC 5 borough area, such as Staten Island's GreatKills (Haven't been there yet), Brooklyn's Floyd Bennett Air Field (The best thus far), Flushing Queen's Memorial Field (nearest to my place), and perhaps Far Rockaway beach, but I wouldn't put Central Park or Bronx's Inwood Hill on the list just yet, even though I don't mind a group gathering there.
However, to get dark enough sites for viewing the Milky Way, I would suppose one needs to get as far as Montauk (via LIRR Train - 10c bus - S94 summer bus - lighthouse), CatSkill (public travel option unknown), or some unknown long island location.
After checking out google map, I have reason to believe that the golf/lake park neighboring CT's Foxwood Resort (which has a special ride from Flushing for $10 plus free meal/$55 Casino coupon) could be a dark enough place to spot the Milky Way.