Do you have any particular zoo in mind? There's thousands of zoos out there...
The answer for each is going to vary a lot, as well, by who you're asking. What's well known and popular with the general public, especially for locals, is often going to be different from what people here would say, since we generally know what the really rare species are. It can also change a lot for a zoo's history.
As an example, right now the general public would probably say hippos for Cincinnati, thanks to Fiona. For many of us, it would be something more like aardwolf, since they are the only zoo in the USA with them and have been the only zoo for years now. Fiona's also only been around a few years.
On here, San Diego is much more well known for their ungulates and their platypus.