I have to say the worst movie I ever saw was suggested by my mum, and even she agrees that it was a huge mistake. The Gods Must Be Crazy. It was horrible. There used to be a reviewer on tv that mum worked out that any movie he gave two or three stars to ended up being right up my parents alley. The four and five stars were generally too "arty" for my parents, and the one stars were generally just bad. The reviewer gave the worst movie I had ever seen a three star. Every other critic around was giving it five stars, and talking it up as a masterpiece. In what universe was that movie a masterpiece? I fell asleep. Mum fell asleep. But since we had a three day rental, we tried to watch it again, and again. Each time we fell asleep, though I tried to stay awake the third time. It was then we realised we weren't tired, but it was actually putting us to sleep. Mum can fall asleep at the drop of a hat (the opening sequence of Gladiator springs to mind, you know the huge battle sequence, mum started snoozing about 5 minutes in, then woke up just as the tiger sprang out in the colleseum fight at the end) but I never fall asleep watching movies, unless I am really tired, or it's Peter Jackson's King Kong. I really want to watch that movie, but I always fall asleep. Don't know why. The Gods Must Be Crazy should be used as a treatment for insomnia. Works for me. But even worse they made sequels.
As for other terrible movies, have to agree with those that have said Alexander, and also add Marie Antoinette (Kirsten Dunst). I expected Twilight to be bad just on the basis of the novel being one of the worst pieces of fiction that I have ever read. A friend of mine raved about Rocky Horror Picture Show, but I have never understood the attraction. I will watch up to Sweet Transvestite, then turn it off. From that point I just can't watch anymore.
One of my favourite movies (in my top 5) is one of those so bad it's good. The Pirate Movie. It's definitely a family favourite, and I know every single line of that movie. Kristy McNichol and Chris Atkins in a Pirates of Penzance spoof, filmed in Victoria with Aussie actors. It is hilarious, especially when you see people in it that have gone on to bigger things, and it is a really, really bad movie.