meerkats. Never mind my signature probably makes me biased towards them, but maybe I'm biased towards them mainly because they are so iconic and popular. They just seem to be in a quite a few zoos as far as the states and Europe go. They also seem to be very popular with kids. I honestly cannot imagine not ever being able to put meerkats in my game.
After that I'd say the Black or White Rhino, probably the Black Rhino, and then it's really more varied. Penguins I tend to associate more with aquariums but maybe that's just been in my experience. So given that, are they really popular zoo animals or just popular animals in general? After that I'd say the Cougar (aka Mountain Lion/Puma). Beyond that, there's quite a few animals that are fairly representative of the regions or biomes they are home to, but are they really common in zoos - the African Leopard (who I really want in the game), for example, may be a common thought African animal, but they're not common in zoos.
So yeah I'd probably say meerkats, the Black Rhino and the Cougar, maybe a type of Penguin if they are more common in zoos than I realize.