Definately the "Top of the Rock" (Rockefeller Plaza observation deck)
I was very (very) lucky to be given a private tour last Wednesday (26th Oct) and would recommend it to anyone.
The views are simply stunning.
It scores over the Empire State Building for three reasons;
1: The incredible, unobstructed, view of the wonderful ESB itself.
2: The view north across Central Park (obstructed from the ESB by Rockefeller Plaza!)
3: It's Midtown position - in amongst all of those other wonderful buildings.
It opens to the public on Tuesday 1st November.