The most pleasant surprise in Bilbao's Guggenheim was the Richard Serra exhibition.
Just don't read the explanation: "The Matter of Time allows the viewer to perceive the evolution of the artist’s sculptural forms, from the relative simplicity of a double ellipse to the complexity of a spiral."
See here.

I would say that Serra in Bilbao's Guggenheim is above all the intense pleasure of being a child in a labyrinth.