@Quartz92: just go out and purchase the 2008 book "America's Best Zoo", which provides detailed reviews of 60 zoos, and at the back of the book it ranks the best zoos in 20 different categories. But in reality it is almost impossible to get a substantial list of the 10 or 20 best zoos in the U.S. or North America. Just for example I realize that you adore the Toronto Zoo and despite its deficiencies would rank it as an excellent zoo. I, for better or worse, think that it is a very large zoo that has only average exhibits.
The general consensus is that the Bronx and San Diego are two of the best in terms of visitation, exhibit quality, animal collection, fame, etc, etc. After that it becomes much more subjective.