One of my favorites at the Art Institute....the 15 ft. high Mao Zedong done by Warhol. The way the museum has this set up, you walk around the corner and are taken by surprise with it.
This one was so creepy, it was cool. The Belgian Delvaux's "The Village of the Mermaids" was based on the artist's dream that the motionless group of women he walked passed had turned into mermaids by the time he reached the shore. You can't really see it, but the mermaids are in background (right center).