Speaking as someone who can find worth (if not artistic merit) in just about every film listed in this thread, there are very, very few films that I regret seeing. I may not like a given film, but it doesn't fill me with bathos over those precious lost moments of my life.
But there is one film that iritates the living shit out of me, and that is Blow-Up (1966). It is, nominally, a story about a photographer in the 60's that may have inadvertantly captured a murder on film. This could have potentially been interesting, if it weren't so absolutely fucked in its execution.
The thing about the movie Gerry is that, while absolutely nothing happens in it at all (whatsoever), the film does not try to hide that. Blow-Up, however, is the cinematic equivalent of Coitus Interruptus, the film version of the most evilly painful blue balls one can ever experience. I cannot overemphasize how much I absolutely hate this movie.
Hate. This. Movie.
HATE.
IT.
The film promises *something* and, in the end, delivers *nothing*. Just when *anything* is about to happen, it stops and decides to start something else, as if the plot has some committment issues. This film contributed to me becoming an existential nihilist because I could not believe in a kind, moral, and/or just universe after being forced to watch this piece of effervescent fuckassery for my Art of Film class. Afterwards, I had to punch a wall in a friend's dorm room just so I could feel something again.
I, for the most part, feel that many "funny critics" go too far in dissing artists and their work, crossing the line into unjustifiable personal attacks and slander/libel. But in the case of this film, I can not only ignore it, but I encourage it. Also, I kinda felt a bit giddy when David Hemmings died in 2003 due to the fact that one more person who worked on this fucking movie no longer draws breath.
As it stands, this is the only film I have ever seen (so far) that has driven me off the edge of common decency like this, and I normally don't react to something as trite and trivial as a film with this degree of vitriol, but if I could sacrifice a thousand children to the darkest gods of history to erase this piece of shit from existence... I might give it some consideration.