Anything MST3K is pretty awful, but I'm pretty sure D list movies have no aspirations beyond breaking even. (My favorite, by the way is Space Mutiny, if you haven't seen it, it's the best of the best). But one thing that makes a movie truly brutally terrible is thinking that its a good movie. A movie that svcks, but knows it svcks can't be considered the worst movie, because it doesn't have that arrogance that drives it over the top.
The reason I specifically cited that particular film was because of its use of depictions of real violence just to save money. That goes beyond mere bad, it's tasteless, disrespectful, and utterly disgusting. I'll take a pretentious bad movie over that kind of thing any day.
Regardless, Space Mutiny is hilarious. If only for http://www.youtube.com/watch?v=RFHlJ2voJHY.