I took the liberty of citing Meltwaterfalls' suggestions here so they can be voted for/against.
Here's his citation:

Major Buildings of the Chicago School of Architecture (C i, ii, iv)
The Chicago School revolutionised architecture by essentially placing the 'skyscrapper' as key component of modern urban planning. Chicago is such an architecturally rich city and its main buildings have shaped many cities world wide. There are also several buildings through out the USA that could also be included in New York, St Louis, Buffalo etc. The main architects involved would be Louis Sullivan, Daniel Burnham, Dankmar Adler, William LeBaron Jenney, John Root and to an extent Frank Lloydd Wright but his major works are a sperate entity.
It seems odd that there are no 'skyscrapers' on the world heritage list as they are such an important part of Modern town planning, this seems especially odd as many other aspects of modern architecture have been included yet 'vertical architecture' is only represented by San Gimignano (Italy) and Shibam (Yemen) which date from the 13th and 16th centuries respectively.

If only to generate discussion, off the top of my head I was also thinking of: Amritsar(Ind), Ur(Iraq), Plzen Brewery (CzRep), Irish Round Towers, Olympic Park in Munich

~~Personally I support Chicago, Ur and Amritsar.

I agree with Assif. The others may be interesting but are unlikely to get inside the top 50.

Hi Thanks for posting this one for me I still only have erratic internet access so much appreciated.
I hadn't really thought about the limitations of being in the top 50 it is not really that many spaces when you think about it. As such I will withdraw the Round Towers of Ireland and Plzen brewery and have a think about Munich Olympic Stadium, but with the right nomination dossier pretty much anywhere (le Havre) can get on the list so you never know.