Fred Astaire could dance in an empty room and it would be a masterpiece, so the sets in Top Hat (1935) are just icing on the cake.
I was first taken with this this hotel room, because it is completely unconcerned with realism. There are no windows, a space massive enough for an aerobic tap number, and walls that reach up to the soundstage ceiling with out a hint of reasonable scale.
But this hotel room was swiftly topped by the set of Venice, Italy:
Just to be clear, this is not supposed to be the Venetian Hotel, this is the actual Venice, Italy.