My nomination for the greatest "so bad its good movie" is Vigilante Force, starring Kris Kristofferson, Jan Michael Vincent, and Victoria Principal. This trailer alone will show you what I mean. Some movies are just so bad its hard to stop watching them.
Never watched that one, but definitely another 1970's classic. The thing about Vigilante Force is that I think they were actually trying to make a serious movie, but it was just so over-the-top cornball crazy.
Incubus; 1966, Starring William Shatner All done in Esperanto. The climax of the movie, so to speak: Where he has his way with her. It was really baaaaaaaaaaaad.