"Night of the Lepus", or some such title. Dozens of giant, blood thirsty rabbits terrorize a small town and eat people.
Gotta be the worst sci-fi movie ever. It's only saving grace is the absurdity of the theme, which compels viewers to watch it.
Another movie so bad, it's a must see is "Motel Hell." Farmer Brown and his frumpy missus operate a motel and smoke house. Out back is a garden with more than veggies planted there. As the label on the smoked sausage says, "All kinds of critters go into Farmer Brown's fritters." Suppose this one is what you'd call a cult classic.
While perhaps not the worst, these are examples of movies so bad, they're actually entertaining.
Les