Worst film of all time... well, are we including the hundreds of films that MST3K has riffed on? 'Cause Monster A Go Go is a pretty freaking' BAD movie.
There's also a strange thing about bad movies. Bad as in so bad they're enjoyable? (Plan 9 From Outer Space) Just so much of a train wreck that you can't stop watching despite the fact that it's just so atrocious? (Battlefield Earth) Just bland and boring with nothing actually happening? (Twilight)
'Cause if you ask me, the worst thing that can happen to a movie is to be just mediocre. Nothing of note. Not so bad that it stands out in badness, yet nothing wonderful, insightful, funny, or anything necessarily "good" about it, even to make fun of, to the point where the film is just kinda... there.