The building suspense as the group, having entered Zinj, are watched each night by the grey gorillas, and eventually attacked, never fails to induce shivers.
The expedition to Zinj must brave a trek through a part of Africa that is undergoing a civil war (and apparently a genocide). The worst part is that the tribe that is performing said action has a well-recorded history of cannibalism.
Once the expedition arrives to Zinj, it's out of the frying pan into the fire: finding what little remains of the previous expedition, then being within a Closed Circle surrounded by a Killer Gorilla tribe, then running away through the jungle in the middle of a volcanic eruption, and then being attacked by a Wacky Wayside Tribe of cannibals...
The opening, where two scientists are attacked by an unseen force, later revealed to be the grey gorillas, especially the part where one scientist has the other scientist's eye thrown at him.
Anytime the grey gorillas kill a group member, particularly the deaths of Richard and Homolka.
I would say Kahega's was worse. It's one thing to have your skull crushed. It's certainly another to be Eaten Alive by a pack of vicious gorillas.