party space?

Ive been to a few bday parties on the roof terrace at The EV Standard Hotel.
At least I think it's the Standard. The newish all glass monstrosity by cooper square. Stellar views and perfect for between 50 and 100 people.

Could also try some arangement at PHD. Penthouse at the Dream Hotel. They won't give you the whole room but certainly a section.

This is not below 14th St, but it's on 5th & 27th: Empire Penthouse, 230 Fifth.

ramscale

http://loftrent.com/lofts

16th street and 3rd avenue:
http://richardlohrstudio.com/

not an apt, but the Soho Grand Hotel offers the penthouse - decorated like an apt - for private events.

What price range are you looking at? I know a really nice loft in Soho that is rented out for occasions.