I'm surprised no one has mentioned Troll 2 yet... although I wouldn't call that my personal best worst movie there is so much more out there to be feared... Italian Batman comes to mind...
I was surprised by that also, DC! Especially since there's a documentary made by the guy who played the kid in Troll 2 called Best Worst Movie.
I have a huge list of these that I refer to when I need a little B-movie pick me up, but here are 10 of my favorite best worst that never cease to entertain me.