Not quite in the intended spirit, but I nominate Dan Brown - for having the nerve to call himself a writer.
Oh come now. He writes utter pap but at least its page-turning pap. Its a bit like watching an action movie - say
Mission Impossible - you know it isn't high art, but you won't get bored either. There is some stuff out there which trying to read is like wading through a bog. Chaucer falls into this category, but its not his fault - arguably Shakespeare does too, but they wrote for a different time and audience. Hell - has anyone here ever tried reading the original version of Robinson Crusoe? It's like treacle. TREACLE.
But these should be exempt in my view - although tough journeys they are rewarding windows into another time. There are people writing now who intentionally make things as difficult as possible for the reader. Brown doesn't do this - sure its not going to be remembered in four hundred years time (probably not even forty) but that's not what he's aiming for.
With this in mind, I nominate T.S. Eliot, specifically for
The Wasteland as a school textbook, but also Robert Jordan for
The Eye of the World, the fantasy epic which fails to be remotely engaging and knowing it is an epic just makes it that much more of a slog.