Best bad book?

Germaine Greer, in The Guardian:

In 1978, a guest at my little house in the Tuscan hills left behind a paperback copy of Colleen McCullough's The Thorn Birds. Having nothing else with which to read myself to sleep, I took it to bed with me. When the clatter of the nightingales (the original thorn birds) gave way to the pre-dawn chorus, I was still reading, utterly engrossed in the best bad book I had ever read.

I was engrossed in The Thorn Birds once too, though I don't think I had the excuse of having nothing else available to read. I haven't looked at it in many, many years. I wonder if I re-read it now if I would find it "the best bad book" I've read. Until I re-read it and find out (if I ever do), I wonder which book is the current winner in that category. What exactly does it mean to be "the best bad book"? The book you like best, even knowing that by some standard it's pretty bad? Bad in what way? Bad writing? Bad politics? (Greer says "It would probably be over the top to denounce The Thorn Birds as a sneakily racist and sectarian book, but it is definitely contrived and insidious.") Bad (improbable?) plot? Bad dialogue? Just a bad idea? Maybe I'll nominate Lady Audley's Secret (it's fresh in my mind because I just finished teaching it). Any further nominations?