Beneath the Planet of the Apes is bizarre but interesting (that's the one with the underground mutants and the cult that worships a bomb). Escape from the Planet of the Apes is the one where Cornelius and Zira are back on Earth in the present (or rather, the 1970s), and Conquest of the Planet of the Apes is the rather bleak, dystopian, violent one with apes oppressed by humans, who then revolt (it also features the original version of the Caesar character). Oh - and Battle for the Planet of the Apes is fairly forgettable, but still worth a look. :)