I honestly think the mantled guereza is underrated. I agree we need more SA monkeys but the mantled guereza is also essential for me. I've seen it in almost every zoo I've been too.
Other than that, there is one primate that is never mentioned and that I really like: the owl-faced monkey. I did not even know it existed a few years ago but then it arrived at Antwerp Zoo and it's now my favorite monkey to see when I go there. It's very endangered and Antwerp has been very successful with them, getting a baby almost every year. So they have a special place in my heart. Not at all essential for the game but would be a nice oddball addition for sure.
Man, I don't even like primates that much. I'm really not a huge primate fan. But we need many more monkeys in the game to build realistic zoos.