The worst one I've ever seen is probably Can Heironymous Merkin ever forget Mercy Humppe and find true happiness?, a film made by, and starring, Anthony Newley in 1969. (Joan Collins was in it as well, and if I'd been her, I'd have fired my agent on the spot.)
A pretentious yawn from beginning to end. Newley was trying desperately hard to make an art film but unfortunately he was no Goddard or Truffaut.
Wolf Creek - So boring it was coma inducing
Funny Games - Turned it off after 20 minutes
Blair Witch Project - Snooze Fest
All the American Pie Films - Just stupid and very unfunny
Eternal Sunshine of the Spotless Mind - What a waste of film
alexander the great (the colin farrell version)
die hard 4.0
armageddon
sahara
jumper
leaving las vegas
grease 2
home alone 2, 3, 4, 5 etc (how many did they make!?!)
horses for courses Brum Lady. I remember when the Clockwork Orange came out and our local council banned it. we travelled 30 miles to see it and I couldn't understand the film at all.
In the 1970's when you went to the cinema to see a film, there was always a 'B' movie shown beforehand, and they were always dire. The worst one I ever saw was called Savage Man, Savage Beast. It was a semi documentary about savage acts committed by animals and man. I still have nightmares about it. One part was a film taken by a tourist of a group of lions. In the corner of the shot you could see another car, full of German tourists. one of them got out of the car and went up to the lions with his camera. The film then showed close up detail of him being disembowelled and eaten alive.
All lovely answers but the worst film of all time is called Possession, made in the mid 80's with Sam Neil. Check it out on IMDB. It was so bad the audience turned nasty and the ushers who usually stand by the exit and let you out with a smile, ran out of the auditorium. Nothing else comes close :)