On his blog Boing Boing Cory Doctorow mentioned in passing that he thought that architecture peaked in the 1920's. He happened to be linking to James Kunstler's blog "Eyesore of the Month."
I'm still trying to formulate a response to that claim, partly because I agree with it, and partly because I'm irritated with the proposition of such a question. 16th century English drama and poetry peaked with Shakespeare--full stop. Jazz age architecture peaked with the Chrysler Building.
People like Doctorow and Kunstler tend to look at the stylistic elements of buildings. They sense in modernism, and its descendants, the decline of concern in designers for visual delight. What they don't acknowledge is the increasing complexity of building function and the often vicious economic pressures at all levels of construction services.