McCarthy is famously tight-lipped on discussing any aspect of Blood Meridian, but he is apparently on record rubbishing the notion that it's an unfilmable book. Not quite sure what his thinking is, because I've not seen too many films featuring all-powerful mass-murdering paedophiles, but he's the boss.
The chronicles of Thomas Covenant are a good shout - quite badly written but great ideas and characters. Often the ingredients for a film being better than the book ( e.g Philip K. Dicks novels).
Converse of this would be Le Guins Wizard of Earthsea. Outstanding book that is often marketed as a younger readers fantasy novel. Everything that makes it great comes from Le Guins writing which was off the charts t the time. Try and visualise the film though and it's hard to see what would make it special.