For me, hands-down, that would be the Imperial Hotel that was completed in Tokyo, 1923. On the very day it was opened, there was another of the terrible earthquakes, demolishing much of the surroundings. But ten days later the telegram finally came through, and FLW learned that his beautiful creation had come through undamaged.
Acknowledged by the Baron who put up much of the funding, "thanks to your genius."
Then this breath-taking work of art was demolished in 1967, although the lobby and reflecting pool were moved to an architectural museum, still there I hope.
Here is the computer recreation, I watch this occasionally just for relaxation and refreshing!
Then just today, I found this video of a home on an island, only an hour from NYC, and the commentator says that Frank Lloyd Wright himself considered this one his finest creation...to me all his work is quite intriguing, this one included, designed 1949 to include a heliport!
Apparently, the original owners never even intended to build the larger house...they only wanted the little guest house! Then when they sold, the new owner discovered the plans for the main house, had the money, an built it...here is the story for that.