I would have to say Godard's Weekend was the most unpleasant, and I had to see it twice.
Skyline was a particularly bad movie
Buck Rogers in the 25th Century (1979)
I wanted to blast Twiki the robot with a laser pistol when I finished watching this movie.
My son made me watch Wanted the other day. It was bad, very bad! Imagine if you can, a mash-up of the Matrix with the Fast and Furious. He told me to keep watching for the final twist. I jokingly guessed it would be an Empire Strikes Back moment but I wasn't wrong. It was so predictably bad. Angelina Jolie and Morgan Freeman always play exactly the same parts, but I thought James McAvoy was actually very good in The Last King of Scotland so my view of him has been reduced considerably.
Someone was telling me about this The Human Centipede film which I don't ever want to see. It just sounds sick.
Buckaroo Banzai across the 8th dimension. It was supposed to be some kind of cult film, but it was soooooo bad. I tried on several occassions to watch it and each time I failed miserably.
The Treasure of the Living Dead (1981) a Jesus Franco masterpiece in which Nazi zombies guard a hidden treasure somewhere in the North American desert.