100 Million BC (2008). Watched it with a friend and at first we were just annoyed at how terrible it was - until it went off the deep end in terms of badness and we wound up laughing til we almost cried.
I think some people really need to double check rotten tomatoes to see where your beloved 'bad' movies really stand. A lot of them are bone fide fresh.
Now for me it has to be Death Wish 3. 6% on the tomato meter which I remember a few years ago it was 0%
Nothing like good ole vigilant justice to brighten your day of the world and humanity.
I was going to say Big Trouble in Little China and Mars Attacks but both of those have relatively good scores. So I guess you can't say they are bad movies. But I could watch those two over and over.