Congratulations
@pachyderm pro for reaching the halfway point of this engaging thread. There are a lot of amazing exhibits that have been highlighted and it's been a delight to see the reaction to your choices. I have been supportive throughout as your selections have been mainly excellent. Anyone comparing it to the similar European thread is not being realistic, as there are probably around 1,000 legitimate 'zoos' in the United States, but in Europe the figure is more like 4,000 'zoos' and so there will naturally be quadruple the number of really top-class exhibits. On this American thread, there's been very few that I'd disagree with (perhaps Indianapolis's Dolphin Dome and the stork aviary at Racine Zoo being the top two), but that's all part of the fun.
I've visited 49 out of the 50 establishments on the list, with only the
Duke Lemur Center in North Carolina escaping my clutches.

However, that doesn't mean that I've seen 49 of the exhibits, as several of the zoos have built impressive new animal habitats since I was last there.
I've personally seen
41 out of the 50 exhibits, all 3 of the "former exhibits" and 2 of the 3 Canadian selections.
I'll highlight 5 of my favourite choices (in alphabetical order):
Kansas City Chimpanzee Exhibit - Kansas City Zoo (when I visited the zoo in 2010 and saw a troop of Chimpanzees come out of their 3-acre forest, it was a magical experience)
Kingdoms of the Night - Omaha's Henry Doorly Zoo (a world-class Nocturnal House, literally underground)
National Amphibian Conservation Center - Detroit Zoo (a building devoted entirely to amphibians, plus two acres of scenic wetlands? Unheard of!)
Ocean Voyager - Georgia Aquarium (6.3 million gallons, with Whale Sharks and Manta Rays. Shedd and Monterey Bay are arguably superior overall aquariums, but neither of them have anything as stunning as Ocean Voyager.)
World of the Insect - Cincinnati Zoo (45 years later and no American zoo has topped this exhibit. That's extraordinary.)