Bougainvillea in Positano
Ok I lied. My favorite exhibit was this Felice Varini geometric projection in the arcade of the Grand Palais. From most angles it just looks like random orange painted shapes but from just the right spot looking down the center of the arcade all the shapes align to form a matrix of perfect circles.
You can see more Felice Varini street art perspectives here including one that projects over a whole village!
Yaacov Agam relief with two images projected on opposite sides of triangle shaped extrusions. As you walk from one side to the other you see one image, a mixture of the two and then the other image. Looking at it head on is kind of like one of those rotating billboard ads at Disney.