I was thinking about this, and at the end of the day, I'm not sure that I do. There are films that I've thought of in the past as silly but enjoyable - Robocop, Starship Troopers, Excalibur, Bram Stoker's Dracula - but right now I'd say that they're actually good films, despite having flaws. I suppose something like Jason and the Argonauts might count, but the bits I like are the good, well-made bits, namely the Ray Harryhausen parts, and I've forgotten most of the rest.
I find it very hard to like much SFF before 1980 or so, with the exceptions of Star Wars, Alien and perhaps 2001. Until that rather grubby, worn-in look was established, a lot of SF just looks a bit cheap and silly to me, so that I can't really get with it even when I know that the stories are good. The exception for me might be The Prisoner, which I remember fondly, but I suspect that if I went back to it, a lot of it would just seem bad.