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 after that, who is next? Burger king, Subway, maybe? There is an element of subjectivity to this, but it should be a consensus about which 5 are the most popular. For example, in the US, the 'big 5' are probably San Diego, Omaha, and the Bronx, but what comes after that? Cincinnati and Denver maybe?