Ninja Bugei-chou

Both are historically significant in the history of anime. Both are among the earliest examples of anime produced for an adult audience. Both are very mature, for late-60s anime, showing much more violence, sex, nudity, and mature themes than other anime of the period. Both are very low budget, and so rely on experimental film-making techniques, to tell the story. 

aesthetically speaking. both are very kami shibai, a bunch of animated stop motion paper clips!