I had a strong urge to put Wuthering Heights on this part of the tag, but I think Dan Brown's The Da Vinci Code wins hands-down. Good choice of source-material, decently drawn characters - but the writing is very very mediocre, and I always get the feeling he's pushing forward too many bunkum theories.
I never understood the fuss about this book - it works as a thriller, but that's about it. It's forgettable and certainly not deserving of so much publicity. Personally, I believe Dan Brown cashed in on subjects he knew would raise controversy, thus gaining the book attention which in turn increased sales. So if a book works better as a marketing strategy than as actual writing, that doesn't say much for the book in question.