Here's the trailer for the first Planet Of The Apes sequel, Beneath The Planet Of The Apes. When I first saw this movie as a kid, it really freaked me out. The mutants flat-out terrified me, and the explosive ending simply blew my little mind and disturbed the hell out of me. Then, for a long time, this was actually my favorite of the films, probably because of the apocalyptic ending and the film's overall nihilistic tone. I also dug the effects of the underground ruins and James Franciscus' more action-driven hero.
Nowadays, I like Conquest best among the sequels, but I'm still very fond of this one (all of them, actually). Enjoy.