Beneath the Planet of the Apes (1970)
Yes, we know – the original Planet of the Apes is probably the better film, but if you're looking for an apocalypse movie, the sequel offers up a double whammy of armageddon action.
Not only is it set in (spoilers!) the post-apocalyptic Earth revealed at the end of the original film, it goes on to (more spoilers!) blow up the already-ravaged planet with a whopping great Doomsday Bomb. More bang for your buck.