I've done 3 of the NY observation decks and this is IMOP the best. Obviously pick a clear bright day. The views are simply stunning. The outside space is huge and even with the crowds you can find a quiet spot to enjoy the views. The whole area around the Edge is well worth a trip so add in the Vessel, the High Line,The Shed, Rock Garden and Chelsea Market for a great time and other than the Edge all the other attractions are free.
Oh and make sure you enjoy the ride down. No spoilers but its pretty special.