I love the Key Tower. Clean lines, nice cap, and an architecture that despite being 200'+ taller, demurs to its more extravagant neighbor (as it should).
One of my favorite Pelli buildings (and I find most of them fairly understated and bland).
BTW, this one turned out much better than its 50 story 700' cousin that was to be built in Hartford at the same time (I think it was even called Society Center as well!):