The qualities that indicate a good zoo to me are the following:
A well-rounded collection that makes sense for the site (size, site quality, climate)
- This can be achieved in different ways, preferably a good spread of mammals, birds, herptiles, invertebrates and fish. If a zoo has dedicated areas to these groups on a taxonomic basis, as long as that representation's there, totally valid. If it's on a zoogeographic basis, again, as long as that spread's there, all that matters. Prospect Park Zoo has a decent spread of these different groups on only 12 acres of land, Philadelphia Zoo has a good spread on 42 acres of land (especially so with the Spiders Alive exhibit, though the lack of venomous snakes is thankfully temporary), and Saint Louis Zoo represents all these groups very nicely on 89 acres. Specialty parks (e.g. San Diego Zoo Safari Park) may lack in some aspects but can also more than make up for it in other aspects (rarities like platypus, specializing in hoofstock/an open-range safari experience), plus it helps when the sister collection is the world-famous San Diego Zoo.
On the flipside, specialty facilities (e.g. aviaries, reptile parks, native wildlife parks) are also highly engaging to the general public, just within a different capacity than what people may expect coming from the "idea of zoo". Which leads into my next point...
A clear objective/mission statement
- A zoo has to be clear in its goals and not be muddled by conflicting motives. For example, Houston Zoo's slogan is "See them. Save them". Simple, concise, effective. Philadelphia Zoo, "Where the Zoo Moves Around You". The latter is moreso highlighting its design philosophy, rethinking the "idea of zoo" if that makes sense, which in turn engages people and gets them to think critically about wildlife conservation. An example of what not to do (in my opinion) would be the Aspinall parks. I actually came around to the idea that there is value in rewilding megafauna like gorillas (if the wild-born offspring from the Beauval-born gorilla and the Aspinall-born gorilla is anything to go by) and tigers, elephants, the issue is much more nuanced than that. That being said, the oversimplistic rhetoric espoused by Damian Aspinall effectively undermines what otherwise could be an engaging way of communicating a conservation message, as it comes across as pretentious, underhanded, condescending and redundant.
Past that, zoos are vital community assets that provide outreach programs to schools, engage other parts of the community with conservation initiatives and even provide venues for events that would give money back to the community.
This article here highlights how Memphis Zoo provides that community support.
https://www.commercialappeal.com/st...-master-plan-financial-struggles/70924460007/
Quality/consistent exhibit design
- This one's my personal favorite to talk about. I study zoo exhibit design as a hobby. One of the things I like about zoo exhibit design are the ways that the animal interacts with its surroundings, be it vernacular architecture or a landscape-based design. I especially like the synthesis between a *good* habitat and a general park-like nature, such as Zoo Antwerpen or even Pittsburgh Zoo. The trick is having a consistent design philosophy, in the sense that the general public should feel like they are in the same habitat as the animals, and associate animals with nature rather than vernacular architecture. Jon Coe, one of the greats in zoo exhibit design, had kids draw what they saw at Woodland Park Zoo, and the kids drew gorillas in trees or giraffes on a grassy plain, rather than gorillas in a cage or giraffes in a chain-link fenced yard.
https://www.researchgate.net/public...Drawings_New_Tools_For_Zoo_Exhibit_Evaluation
So overall, one species does not a great zoo make. It's the overall collection, mission statement/objective and design philosophy that makes a zoo truly count.
Unless it's elephants. 
