My initial reaction was "what's so special about Chicago's collection of pre-war skyscrapers?" because lots of other cities have historic skyscrapers that are even more notable than the ones listed here. Detroit comes to mind, specifically. However, when digging in a little further, it's pretty remarkable that all these buildings are from before 1900. That's significantly older than Detroit's famous skyscrapers, and ones that come to mind for Cincinnati, SF, Pittsburgh.
If we can expand just a couple years after 1900, I'd nominate Cincinnati's Ingalls building. It opened in 1903 and is the first reinforced concrete skyscraper in the world.
I'd also nominate the Central Trust Building, but it's from 1913, which is stretching the time constraint a bit too far, I think. But it's a badass old skyscraper nonetheless.
