Well, I have so many favorite baaaaad scifi films (emphasis intentional) it's hard to choose.
One of the top worst films-
The Creeping Terror, 1964- so very bad, so very campy, hard to believe most of the contemporaneous audiences didn't have access to decent drugs so they could truly appreciate this film. The monster is an old carpet.
The best intentionally bad film-
Big Man Japan, 2007- mockumentary with some of the best monsters ever. Beats The Attack of the Killer Tomatoes hands down.
Old films- special category-
Film that scared the be jabbers out of me when I was a little kid and watched it the on black and white TV in the late 50's- Invaders From Mars, 1953.
Film my mom turned me on to as a kid- The Boy With Green Hair, 1948. Maybe not quite science fiction, but definitely worth watching.
Best film, quite a few could make this cut, but the film Blade Runner is one I can watch over and over.
Oddest film, Tetsuo, The Iron Man, 1989. I like it. I'm not sure why, or what it says about me- but cool film
OK, I could go on- but these are the ones that sprang to mind tonight.