My top ten - I haven't been to all of these (I'm still in high school) but I know enough about them to rank them.
1) San Diego Zoo - Unbeatable Current and Historical Collection, Newer Exhibits Some of the Best of their kind. Unbeatable List of Record Breakers.
2) Bronx Zoo - Great Collection (I wonder if their rank will falter with the economy)
3) Omaha - Awesome Collection, Average or Above Average Exhibits, Lots of Expansion
4) Columbus - Large Collection (Mostly Reptiles, Fish), Average or Above Average Exhibits, Numerous Record Breakers, The Fastest Expanding Zoo in the Nation
5) Miami - Impressive Collection, Great Barrier Free Exhibits
6) Wild Animal Park - Great Collection, Awesome Exhibits
7) Woodland Park - Above Average Collection, Many Exhibits Best of their Kind
8) Lowry Park - Maybe Some Shady Dealings but Great Collection, Average Exhibits
9) Minnesota - Average Collection, Outstanding Outdoor Exhibits
10) Detroit - A Recent Contender for My List, Average Collection but Exhibits the best of their Kind (Mixed Primate, Red Panda, Aardvark, Polar Bear, etc.)
Next on the List Would Be: White Oak, DAK, the Wilds, St. Louis, Portland, Atlanta