The absolutely WORST movie, hands down, is “Plan 9 From Outer Space”. The special effects were breathtaking . . . from the actor knocking over a cardboard tombstone (and kept in the final cut) to the “flying saucer” — two paper pie plates doused with lighter fluid, set afire, and thrown in front of the camera. The classic scene where Gregory Wolcott (hero) and Mona McKinnon (heroine) watch the alien craft land and are looking in two different directions. And Bela Logosi, who died after filming started — the director kept his scenes in, but hired an extra as a stand-in who was a full head taller and walked about with his caped arm in front of his face! Great production in glorious black and white.
A runner-up would be “Blood of Dracula's Castle” (John Carridine’s last movie), The trapped, scantily clad victims are chained in the dungeon (presumably to be tapped for blood) and some of them are standing on film boxes (with the movie title clearly shown on them)!
Plan 9 gets a bad rap and it's easy to mock, but I'd argue that the worst movie is one that bored me, and whatever you want to say about Plan 9's production quality (or lack thereof), it's not boring. It moves pretty quick for its 79 minutes.
Hell, Plan 9's not even the worst Ed Wood movie.
Johnny Depp in “Ed Wood” is great. Especially in the scenes directing Plan 9.