For Detroit:
Unite d'Habitation. I'd like a Unite by Le Corbusier to have been built south of Mies van der Rohe's Lafayette Park.
Ministry of Health and Education Building by Lucio Costa in Rio de Janeiro.
Guarantee Building by Louis Sullivan in Buffalo.
Inland Steel Building by SOM in Chicago.
And as much as Rockefeller Center as I can get.
Honorable mentions include Park Hill in Sheffield and Robin Hood Gardens in London, but both are very site dependent so I stuck with more universal buildings.