This is a tough question; I think the original Planet of the Apes series is best viewed as one epic story. However. In terms of overall quality, Escape From The Planet of the Apes is probably the best-- but I hesitate to vote for it because I can't forget that they expect us to believe that Cornelius and Zira roamed around the Forbidden Zone, somehow found the lake where Taylor's ship sank, somehow knew it was there, somehow raised it to the surface, somehow repaired it, somehow fueled it, somehow learned how to fly it, somehow launched it and somehow found their way back in time to roughly the same time that Taylor left. That's a lot to swallow. The last two movies are ostensibly the worst in terms of production values and story, but Battle has that great speech by Roddy McDowall. Beneath has production values equal to the original, but the story is weaker and I always felt that the mutants were superfluous to the concept-- although I did like the climactic sequences in the mutants' lair. And, as noted, there is Linda Harrison. If forced to choose, the far-fetched premise of Escape forces me to default to Beneath.