Big FLW fan here. A few years ago we made the trek out to western PA for a weekend trip on the motorcycles, and saw Kentuck Knob and Falling Water houses in the same day as they're fairly close to one another. Both great, but Falling Water is on the highest level of human achievements in my book; more awe inspiring to me than the great cathedrals or pyramids, it feels like all of humanity was lifted when that house came into existence in the 1930s, I'm not kidding. I can't recommend it strongly enough. If you go, take one of the longer tours.