largest zoo

What 5 zoos do you consider to be the 'big 5', whether worldwide or within a specific region. By 'big 5', I mean, as the name implies, which 5 are the most prominent/major institutions. For example, within fast food restaurants, it's probably undeniable that Mcdonalds is part of the 'big 5', but...

"The Omaha zoo has received another big accolade: “largest” in the world. Our Henry Doorly Zoo & Aquarium is not the world's biggest zoo in area, but the travel website touropia.com ranks it as the largest by combining geographic size with number of species. It's an unusual methodology...