Boring's the worst, other than things which are just plain offensive. As you said, bad quality works can just be used as a laughing stock, but when something's boring that obviously implies there's no interest to be had there at all. Even something like a serious documentary wouldn't count as boring if you're interested in the subject/information, so for something to really be boring is an impressive (or should I say unimpressive!) feat. However, worse than being boring is something that's offensive. I've yet to come across anything that offensive so far other than one or two of Frankie Boyle's less PC routines, but I'd much rather sit through something boring rather than something offensive. Although since the point of entertainment is to entertain, boring works really should be bottom of the barrel.