Were spoiled that's what it is. Plenty of decks.
30 Hudson, ESB , Top of the Rock, 1 WTC, Vanderbilt in the future (don't forget about Vandy!) ... the Vessel (should count as one). Also some wicked places to get some views, like if one is willing to walk on the GWB (best skyline view IMO).
Over in NJ, we have some natural cliff side observation ledges. Union City, above the cliffs, is a great location.
And if one is adventurous, take a helicopter ride over the city. Best bang for the buck. Best 20-30 minutes one will spend for a mental imagery that will be ingrained in ones mind.
Plenty of ways to view NYC. From the air, and from the street. From roof top bars, to pretending to stop for repairs on the NJ turnpike to take in the views. On a side note, if one is to stop for "repairs", with thier flashers on, Chris recommends this spot right here for "repairs"...
https://www.google.com/maps/@40.7449...7i16384!8i8192
Eagle Rock Reservation is also worth a trip if one is in NJ. On a clear day, it will make your jaw drop. Also if one is cheap (free on weekends), just go to Newark, near NJIT, and go on the weekends to the NJIT parking deck. Go to the top top of the deck, and enjoy the view. Newark NJ, right by Branch Brook Park, has one section that is elevated way above sea level, and provides a unique angular approach to the skyline. Staten Island, Statue of Liberty... so many ways to view the skyline.