For the actual question of the thread:
1. Duck
2. New World Monkey
3. African Dwarf Goat
As the 3 true highest impact animals arnt available, here is of those 3.
1. Coati wins cause its basic and common and SA desperatly needs basic and common to have a solid base to build upon with more specialised animals.
If monkeys are SAs antelopes, the main group with high numbers and representation in zoos especally regarding many species in the same zoo, then coatis are its meerkats, a small charismatic species that is in pretty much every zoo and fits everywhere. An african area without meerkats does not feel complete, same for a South american area without coatis
2. Big big gab down to the tree kangaroo, for the simple reason that its the hardest to do justice as a mod of the 6.
Its a vital piece to represent bew guinea and while i really really really want the victorian crowned pidgeon and short beaked echidna along side it, it alone would be enough to make tropical oceania go from just cassowary to something.
3. Baboons are cool and iconic ABC zoo animals that are actually common in zoos, something that cant be said about the other 3.
And now while i didnt pick the other 3:
Muskox: Completes tundra and thats it. Otherwise just another ungulate in a sea of ungulates, meaning its an easy mod and only plays a role if the player wants a tundra area, which is valid but much more niche then any continental area.
Walrus: Same as Muskox, but less essential as having 3 very different ungulates from each other is nice variety and important filler, but 3 different piniped pools is much rarer to find in the same zoo.
The gray seel and the californian sealion (very common in arctic themed areas in a real zoo, they are more common there then most arctic animals) allready fill its gameplay niche and while representing all 3 piniped familys would be cool, i wouldnt call it nessecary, especally not above any more important animals like new world monkeys, petting zoo animals or ducks, which need some base line representation badly.
Secretary Bird: Let me tell you a secret. I really dislike the Secretary birds popularity and it doesnt make sense to me.
I can understand that the mighty amazingness of ducks isnt something everybody agrees on, but what about a pelican?
Why isnt the literal ABC zoo bird next to the ostritch and flamingo the most popular Bird?
I can get niche preferences, i can get that the whole actually wanting to build for animals you see in zoos seems to not be something everybody agrees on especally regarding more "boring" yet common animals like ducks and ungulates, but come on why isnt the super common water flying bird pokemon a top contender?
Why arnt people rallying around the big stupid bird with a big silly mouth sack that looks unique, metall as hell and is in allmost every zoo in some form?
Secretary bird is cool and all, but its not even in my top 10 bird list, far below many things but especally a pelican (and ducks but yk that allready)