I think the Minnesota Zoo is in the 500 species club as well (lots of fish). Building off of the list put together by
@lintworm , I thought I'd attempt to categorize some of the top "collection zoos" in the US. I'm trying not to go strictly by the indicated species numbers due to the massive differences in off-show numbers and because some types of exhibits might give different perceptions. For example vivaria in a reptile house often seem to do a better job of featuring reptiles than do vivaria added as accents in an elephant barn. Feel free to add your thoughts.
The big four: collections that stack up against the best in the world and are good/great in several categories.
- Bronx. Best small mammal zoo in the US, top tier for ungulates and birds. Biggest collection hole is marine life, but a smattering of freshwater aquaria ensure that fish are represented.
- Omaha. Most well-rounded in US, with best marine collection of any standard zoo. Worst category might be birds, but even there the collection is very good, just doesn't get the featured treatment that everything else gets and is harder to appreciate because many species are free ranging in the big halls.
- San Diego Zoo. In the conversation as best zoo in US for primates, birds, and reptiles, and has excellent carnivore and ungulate collections as well. Fish are the big weakness, but perhaps the new Children's Zoo will help rectify that when it opens.
- St. Louis. Perhaps best in class for insects. Also very good in general for mammals, birds, and herps. Like Bronx and SD, not much in the way of fishes.
Other notably diverse general collections: high diversity and well-rounded, but not quite to the level of the four above (even though some may have more species than ones listed above).
- Brookfield. Good balance, with every taxonomic group receiving some representation. Insects don't get a feature exhibit but are fit in here and there.
- Cincinnati. Vies with St. Louis for top insect zoo, also very good for most mammals and birds.
- Houston. Massive bird collection, pretty good collection of everything else too.
- Memphis. Small mammals and carnivores stand out.
- San Antonio. See Houston.
- Toledo. Surprisingly good for small things.
Formerly top tier collections: exhibit redevelopments and closures have seen these zoos reduce collections that were comparable to those above until recently. Note that this isn't necessarily a bad thing when coupled with improvements to spaces for animals.
- Cleveland. Most notable is the scaling back of the number of species held in the Primate/Cat/Aquatics building. The elephant and tiger redevelopments also utilized space that formerly went to other species.
- Denver. Redevelopment took out a big chunk of the formerly massive hoofstock collection, and now the bird house has just closed.
- Lincoln Park. Lion house just closed for redevelopment, and the hoofstock areas have experienced general depopulation over time.
- Milwaukee. Small mammal house holds fewer species than it once did, and elephant redevelopment displaced a couple of exhibit areas.
- National. Lost a big chunk of the large mammal collection when Asia Trail was developed, and the invertebrate house closure hurt the diversity on display as well.
- Philadelphia. Has cut perhaps a third of the mammal species in the past decade.
Notable in one broad category: I'd argue that the above groups of zoos include the clear top zoos for primates, carnivores, small mammals, birds, and insects, but in a few categories other zoos not already listed are at or near the top of the heap.
- Ungulates - SDZ Safari Park; Miami
- Herps - Atlanta; Fort Worth; Los Angeles
- Aquatics (in a zoo) - Pittsburgh
Top aquarim collections:
- Dallas World Aquarium. OK, more of a zoo than aquarium but great bird collection either way.
- Georgia. Best collection for star power (big sharks, marine mamamls, etc.).
- Monterey. Tops for open ocean and temperate seas.
- Shedd. Clear number one for overall scope and diversity, covers everything well (freshwater and marine, tropical and temperate, exhibits 4 marine mammal species too).
- Steinhart (Cal Academy). Best explicitly educational collection.
- Tennessee. Unparalleled for freshwater.
- Baltimore and New England have very strong general collections as well.
Really good zoos not listed above:
Arizona-Sonora Desert Museum, Columbus, Detroit, Minnesota, North Carolina, Oklahoma City, Sedgewick County, and Woodland Park are all top shelf zoos that fall more into the category of "exhibit zoos".