People who visit Omaha's zoo from out of town are generally stunned to discover the world's second-largest free-flight aviary and the world's biggest desert exhibit (inside the world's largest glazed geodesic dome) and America's largest indoor rainforest and America's biggest cat complex and the largest zoo aquarium in the world — all in one place.
You can see it via a choice of tram, miniature railroad or Skyfari treetop chairlift.
The Henry Doorly Zoo also has an innovative "Kingdoms of the Night" exhibit in which day and night cycles are artificially reversed to show nocturnal activity patterns. The nocturnal exhibit (yeah, it's the world's biggest) has a canyon, a wet cave, a dry bat cave and the world’s largest indoor swamp.
For those who have the time, there's also Pachyderm Hill, the Insect and Butterfly pavilion, the separate gorilla and orangutan exhibits and an IMAX Theatre.
TripAdvisor's complete ranking of hundreds of zoos is here.
1. Henry Doorly Zoo (Nebraska)
Henry Doorly Zoo's desert exhibit, largest on earth, competes
with the zoo's world-class cat complex, aquarium*, rain forest,
aviary and nocturnal exhibits — all either largest in the world
or in America. Just reading the zoo's Wiki entry will make
your eyes glaze.
*The aquarium is the largest inside a zoo, not considered to be
an independent attraction.
2. San Diego Zoo (California)
3. Loro Parque (Spain)
4. St. Louis Zoo (Missouri)
5. Singapore Zoo (Singapore)
6. Chester Zoo (United Kingdom)
7. Prague Zoo (Czech Republic)
8. Tiergarten Schoenbrunn—Zoo Vienna (Austria)
9. Bioparc Valencia (Spain)
10. Gramado Zoo (Brazil)