This is always a subjective question (most lists usually have "Plan 9 From Outer Space" as champ), but I submit to you...
"Manos" The Hands of Fate.
If anyone has seen this, it was probably on the famous Mystery Science Theater episode. You can't imagine how impossibly bad a movie that actually got released in theaters can be. It looks like your Uncle Walter's Super 8 family films from the early 60's. In fact, it was a Texas fertilizer salesman's one and only effort in cinema. Look it up on IMDB.
It is also perhaps the ONLY film in existence that NO ONE can be linked to Kevin Bacon, in even a million steps.
"Manos" The Hands of FateOh dear GOD! LOL! I'm forever scarred by that movie as are Joel Hodgson, Mike Nelson, Kevin Murphy, and everyone else who ever laid eyes on it! Get ready for this: I actually witnessed an online discussion saying that "Manos" was actually a good movie, and that MST3K had edited certain "good" scenes. Yeah, right!
In addition to that "nugget" of film, I also submit the MST3K classics "Skydivers" and "Hobgoblins." Those two may actually beat "Manos" in crappiness.