All of the Harry Potter books by J. K. Rowling. For years I had to hear about how awesome the books are and how I must read them, but when I finally sat down and did so it was a letdown. For something that is supposed to be the best-selling book series in all of history, there are just too many plot holes and lazy writing to agree. It's no use trying to argue the point with Potter fans either as they are all so obsessed with the boy wizard that they can't see any wrong with the books.
Twilight. The movies are absolutely terrible, but since the books are trash I guess it means they accomplished their mission. The Fifty Shades of Grey movie franchise has also pulled out all the stops to equal the books in how cringeworthy they are.