who had the greatest impact on 20th century architecture?

I vote for this guy, Willis Carrier, who made the most significant contributions to the practical air conditioning of buildings. His critical observation that "air treatment" involved controlling the humidity levels in air made it possible for his company and others to design modern HVAC systems. Without him, the modern architects would have had clients who ended up dying like flies in their glass and concrete creations.

Air Conditioning, as we like to call it now, also helped create the sunbelt devleopment in the United States. Florida, Georgia, Southern California and a lot of other places wouldn't be as popular as they are without effective air cooling systems.

But, we should be aware of the hubris that a powerful technological system like building integrated cooling can lead to. If the comfort of a building can be guaranteed by throwing money at it to maintain a comfortable interior, then common sense can suffer. And it has.