Probably Star Crash. WORST MOVIE EVER. Never even heard of it til someone made me watch it. Its absolutely horrible! Here is a quote I found a while ago concerning the movie.
J.C. Maçek III of WorldsGreatestCritic.com wrote, "If Starcrash had a much bigger budget, a better script and fresh ideas that weren't mined from other, vastly superior franchises... it would still suck. That's right, folks, it has to aspire to suck."
Never watch unless you need something stupid to laugh at.
I would cite films such as Troll 2 or The Room, but those pictures actually have tremendous (albeit unintentional) comedic value to them. So much so that they actually manage to flip back around to being some of my favorite movies. No, the worst movie that I've seen has to be Meet the Spartans. When parody is done right, I freakin' love it, but this movie just scrapes the bottom of the barrel by providing pop culture references for the sake of providing pop culture references. Such a lazy film.
The worst movie I have seen, is: The Legend of Zelda: The Hero of Time. It was a real, a little long movie, but short after it was released, Nintendo prohibited it so it is hard to get to see. It had sooo bad acting, so bad effects! A really cheap movie though. And Link had a mom kokiri who was adult!? Kokiris cant be adults, so I hated it.