I can happily say that the only one of those movies I've actually seen is Plan 9 (which I love, in it's own cheese horrid way).
Still, I voted for Highlander 2. Because without my even seeing it, it ruined the memory of a film I loved. Before it came out I read a synopsis of the movie on the net. Somebody claimed to have read the script, and posted a basic outline... this happened a lot, but sometimes someone would just post some BS or off season April Fools, just to get people riled up. When I read storyline for Highlander 2, I just figured it was a joke. Then when it came out, my housemate saw it, and came back babbling the same horrors. Ug.
The only other time I read a real synopsis and figured it must be a joke was for Star Trek V. The short form was "They meet Spock's previously unknown brother, who takes command of the Enterprise and takes it to the center of the galaxy where they meet God." Ya. Right.