The Da Vinci Code.
The writing is mediocre. The characters are utterly unbelievable. The plot is ridiculous. Yet the book is the best-selling adult novel in years: why? It is a decent chase/treasure novel with little puzzles to solve, but that can’t possibly be the point of interest for so many millions of people.
The “sacred feminine” is brought up repeatedly in the novel, and yet no one I talk with who has read the book seems to care. Indeed, it is just the conspiracy of the Catholic Church that people talk about (ironic in this case, because the Church is pointedly exonerated by Brown from any involvement in his plot, and indeed is portrayed as a source of inspiration and comfort.)
Grail legends, lost treasures, and vast conspiracies will always be popular, but I don’t think they are enough to make this a runaway bestseller, especially considering the bad writing. It is beyond me.