I don't know if it's THE most boring but the Gormenghast trilogy has got to be one of the worst I've read I don't think I got more than a few chapters, he spends pages describing a hand gesture or a look, very boring! As for the dune books I Really like them but I will admit they are not the easiest books to get into!
Das kapital to me is one of the most boring books in existence. Everything I have ever read by Marx turns my brain to slush. I guess I just can't quite get into economy books. A close second is Dianetics, the books just seemed to repeat its gibberish over and over, while asking me to give money to go to workshops.
No offense to anyone but reading the Bible is boring specially the Old Testament and the old English version. The modern version is a bit better read.
Manuals for any equipment can be very boring too, you have to sift through needless information just to get to one sentence.