The most boring film I ever saw (partly, I turned it off out of sheer despair) was a porn product called
Den Kåta Honan (Swedish, would translate as "bitch in heat"), which was as interesting or arousing as watching three butchers beating on a piece of meat in an endlessly repetitive way against a sterile background. It was in fact so utterly boring that I remembered the title of the film, in case anyone would ever ask me the question posted at the beginning of this thread
The other porn film I saw was amusing, so I do not want to put down the entire adult movie industry.
The most boring art film I ever saw was called
Color of Pommegrenades (1968) by Sergei Parajanov. It was very hard to get through because it spoke mostly in images and poems that were totally inaccessible, a knowledge of Armenian/Kaukasian culture being necessary to get the gist of the film. I wouldn't say it was a bad film, just that I found it hugely boring.
I actually liked many of the films mentioned in this thread, even
the 13th warrior, because I had read the book as well as the Beowulf and some of the Arab writings it is based on (I thought Banderas was not brilliant though). It just goes to show how tastes differ.