Movie critics have unanimously voted "Plan Nine From Outer Space" as the worst movie of all time. It was an alien invasion sci-fi flick, from the '50s.
Bela Lugosi died, during the filming of the movie and they had to continue shooting, without him. Some of his scenes were shot from behind, with a double and scenes of him leaving his home and walking through a graveyard were cut into the movie time after time.
"Mars Attacks!" ranks #2 on the list.
My personal contribution to the bad movie festival is "Four Eyes and Six-Guns". The only reason that I own a copy of this movie is that Mark and I are in it, as well as my ex and a lot of our friends from Tombstone.