From a photographic perspective, the best time of day to propose is between 30 minutes before and 1 hour after sunrise, or 1 hour before and 30 minutes after sunset (e.g., if sunset is at 8pm, then the best time for your proposal photography would be the period from 7pm to 8.30pm).
Having said that, with my many years of experience in outdoor photography, I can make it possible to shoot at any time of day or night. Just choose a time that works for you and let me know.
From a city perspective, it depends on your own preferences. Do you want the world to see you propose, or would you want a more private proposal? If it’s the latter, then it’s better to organize it to take place on a weekday before 9am — you’ll likely have NYC for just two of you [show example picture]. If you prefer to have people to cheer you on when you propose, choose any other time! The city of New York is there to support you!