I think its obviously Battlefield Earth for large production movies, I really can't understand how a lot of the more popular movies could be the worst you've seen unless you really don't watch a lot of movies, I could go through the whole list, but I'm being lazy right now.
As far as the overall worst movie I have seen, I would have to say "Santa Claus Conquers the Martians" I actually got this in my Christmas stocking from my MIL appearently she picked it up from the dollar store as a gag gift. Well unfortunately I soiled my dvd player with it (yes it's on dvd now) and it wasn't even the mst3k vers. I really don't know what the moron who decided to rerelease this was thinking, but he/she has done us a great disservice. If you haven't seen this then I don't think you really know what an awful movie is. You all would probably change 90% of your answers after seeing it.
Oh and I haven't seen many of the SciFi Channel filler crap as peoples answers, but there are so many bad shows on that station, lightyears beyond the likes of Kevin Costner, Van Damme, Kevin Smith etc. etc. in Craptasticness, I would say the majority even surpass the most Whoopi Goldberg movies, but I am probably wrong there since I stay away from those. I digress.