Beneath the Planet of the Apes is the first sequel to the iconic science fiction saga, launched in 1968. The plot follows a new crew of humans in search of their colleagues lost in the first film, and addresses again the struggle between the civilization of apes and a variant of human beings who communicate telepathically and live underground. Inferior in almost every sense compared to the original, it is a perfect example of everything that can go wrong with a hastily made sequel: it is less ingenious, profound, and spectacular, diluting the powerful ideas of its predecessor in favor of banal entertainment.
Escape from the Planet of the Apes is the third film in the original Planet of the Apes saga, started in 1968. Often considered the best sequel in that stage of the franchise, the plot follows the original ape protagonists, Cornelius (Roddy McDowall) and Zira (Kim Hunter), who escape the destruction of their planet in a human ship, and end up back on Earth in 1973, subjected to the scrutiny of the human race. Although it does not rise as high as the original, it is a worthy sequel that tackles themes such as collective guilt, intolerance, and repression.
Conquest of the Planet of the Apes is the fourth installment of the saga in its original continuity. Set in the (then) near future of the 1990s, the film portrays a society in which apes have been enslaved by humanity, until one more evolved than the rest instigates a revolution that represents the fall of the human race. In addition to closing the line of events leading to the story of the original, this film also shares several narrative elements with what would be its reboot: here we meet Caesar, played by Roddy McDowall, and later by Andy Serkis in the new movies.
Battle for the Planet of the Apes is the fifth and final installment in the original cinematic saga. The story is set after Caesar (Roddy McDowall) has wrested control of the planet from humans, with whom he seeks to maintain peace despite treating them as second-class citizens. By far the least successful film in the saga, it was its nail in the coffin for decades: after its low quality, years would before an attempt was made to tell a story in this universe again. However, it is the film that closes the story of the classic saga, so it is essential if you are a fan of the first film.