I'm a bit biased because the Golden Lion Tamarin is my favorite zoo animal, but looking at zoo presence and the OG grouped wishlist, I would say that a tamarin is the most vital monkey by far. I'm hoping they'll add ~3 at once, but one with mods for others would be way better than none.
Then, I still think a howler or spider monkey is critical to bring South American monkeys to a passable state.
With the lemurs, great apes, and baboons, Africa has several options for primates, but I agree there is a very noticeable absence of smaller African monkeys, which have been very prevalent in zoos I've attended. I think the popular guereza would be a great option for this, and I hope to see it in an African Safari or rainforest/jungle pack. Just behind the tamarin and howler/spider monkey, I think a guereza is the last critical primate pick to make the monkey lineup not obviously awful.
Beyond that, monkeys have a lot of important species remaining. Additional tamarins, the other of the howler/spider duo, squirrel monkeys, and even a tufted capuchin are all pretty important to build a basic lineup for monkey houses. Africa has plenty more to offer, including the De Brazza's guenon, Wolf's guenon, patas monkey, vervet/grivet monkey, Diana/Roloway monkey, Schmidt's red-tailed guenon, collared mangabey, and gelada. Europe is still missing its only monkey, the Barbary macaque. The golden snub-nosed monkey is very important to the large Chinese playerbase who isn't as vocal here. India is still lacking a monkey, and the grey langur and lion-tailed macaque are both strong choices. Even Southeast Asia has more cool primates, including the smiling crested macaque, Javan langur with its golden morph, and the beautiful red-shanked douc. Of all of those additional options, I would prioritize ~2-3 more from South America, ~2 more from Africa, the golden snub-nosed monkey for China, the Barbary macaque for Europe, and at least one for India, assuming we get past the first tamarin, howler/spider monkey, and guereza.