Of books I've finished, The Da Vinci Code. It's been a long time since I read it, so I can't recall specifics but I do remember the moment to moments of the plot being contrived and stupid, and the writing to be bland and simplistic.
The only reason I read it was I was stuck somewhere without a book and I found a copy of The Da Vinci code that had fallen behind a shelf. I figured it was super popular so there must be something to it as I slogged through.
As memey as the whole theory is, I like to mention that George Lucas was influenced by a lot of classic scifi. He was almost certainly aware of the famous Foundation novels. In the second book, the comedy relief character turned out to be the secret villain leading the evil empire and who had mind control powers.