Re: Two on the list are the worst.. one, maybe not.
I believe Plan 9 from outer space is generally reckoned to be the worst scifi of all time.
If we include the excremental Uwe Boll disaster Alone in the Dark, then Plan 9 is far from the worst. But Alone is really horror / fantasy, not SF. I don't know if Boll has ever made a film that could comfortably labeled SF.
I'm sure I've seen SF films on MST3K that are comfortably worse than Plan 9 by pretty much any measure. Certainly less entertaining.
If we include made-for-TV movies shown on the SyFy cable channel, then Plan 9 is closer to Oscar territory than "worst ever".