I had worked whit pubblic in the zoo so
1) Lion
The people come to see the Lion. The big maned one. The roaring. And then when they see them start to complain bout how they re lazy and ask if you drugged them. They will learn and see other magnificent animal. But they come for the lion. Mandatory.
Tigers are an equally accepted alternative, though..
2) Giraffe
The guests know surely only 5 grass eater. Hippo, Elephant, Giraffe, Zebra and Rhino. They know that Hippo are aggressive and Rhino rare so they will not complain a lot of you don't have them. The zebra are horse who wore Juventus jersey and nobody care nothing more apart bad Juve joke. Elephant are an ethical and logistical issue so nobody will complain a lot bout them assence. But Giraffe...they had to see them. And try to feed them. And you can make them pay for this.
You will always have some visitors complain about the lack of elephants. And visitors that complain if you have elephants.
Zebras are a mandatory item of the standard "African Savannah" zoo exhibit. And yes - giraffe feeding stations are quite profitable - just as most feeding stations.
3) Monke
At least one recognizable monke. Chimpanzee, capuchin, gibbons are ok. Macacque are inferior in their monkeness. Gorilla and Pongo aren't valued as monke, they are big monke. Callitrichidae, baboon and guenons are not funny smart monke. They want the funny smart monke. And try to feed them. Is a Nightmare
It's "monkeY".
4 ) Pygmy goat. Not very obbligatory but are cash grabber animal. A lot of school will pay to do a feeding/didactic farm experience. And no you cannot use a markhor or urial for this.
True. And they make good predator food.
5) The meme. Probably capybara. Child love these rat. Very easily maneageble. They ask also for Quokka and Is very sad tell to them that had to go in Australia.
Or you can send them to Stuttgart or Higashimatsuyama.
6) The occidental meat eater. Bear or wolves. Not truly so mandatory but expecially if a wild one of them had killed someone you will cash grab in the seguent month the amount people that come for sayin g "ah that poor guy"
I don't know where you live, but there aren't that many man-eating bears and wolves here to fullfill such an demand...
Bonus
Theorically a lot of people want to see big snake or big croc. But if they had stayed in a reptile house prior in ther live they will complain bout how boring they are. And if not they will start in that moment