This was last asked back in 2006 in AnswerBank, and there's been plenty of dreck released since then.
With the imminent release of the new 'Diana' film which is garnering almost universal bad reviews (http://www.theguardian.com/film/2013/sep/05/diana-review), which film is your worst...and more importantly why.
Mine would have to be 'Transformers - Revenge Of The Fallen'. Walked out after about 30 minutes.
(Dis)honourable mentions go to Matrix Reloaded/Revolutions (I literally didn't understand anything that was happening) and The Blair Witch Project (by the end, I was rooting for the witch).
The wording might have been a little different, but it's been asked a few times since.
I'll always shout out John Revolta's Battlefield Earth (It's the whole L. Ron Hubbbard / Revolta's performance / Scientology thing). They spent a lot of money on it and it's total dross. Aliens land on Earth and try to take over. The invasion fails. The plot fails. It's all one big fail!
The live action version of Thunderbirds as well. Gerry Andersen's marionettes weren't as wooden as the acting.....
A film I will always regret taking a friend to see was "Alpha and Omega" about romance in cartoon wolves. We cringed all the way through... but to be fair it was aimed at children, and clearly very young children only.
I am a massive Madonna fan, and I too found 'Swept Away' pretty horrific. In fact, if I recall correctly, I didn't even wait for the ship to sink before abandoning that film.
Battlefield Earth...excruciating. I kept thinking, "Didn't anyone realise how bad this was when they were making it?"
Dreamgirls however...I have a sneaking regard for that film, in a 'so bad it's good' kind of way.
Many years ago, Ch4 did a series of "the worst films ever made" and I can recall "The wild women of wonga"(and NO, not those ghastly little puppet loan sharks). There also was "The Killer Tomatoes" and also "The hands of Orlac". The last one was about a concert pianist who needed a hand transplant(!?) and was given the hands of a mass murderer/strangler. OMG, it was sooo funny but meant to be scary. Also, The premature burial with Ray Milland. All of them equally awful.