For me it would have to be any of the books in the "Flowers in The Attic" series by V.C. Andrews
The first book was pretty meh but each book that followed got worse and worse, and my sick need for closer made me power through them.
The woman can't seem to make her characters evolve at all. The way they are written when they are 11 are the same way they are written when they are 36.
She loves ending ever sentence in exclamation points! To the point were they lose all effect.
She also writes every character as a complete whore or a sick stalker who can't move on with their lives.
She also falls into the trap of making characters impossibly beautiful, every single character needs a 9 sentence paragraph describing how wonderful they look. This would not be that big of a deal but they do it 15 times for every character. If you took out character descriptions all her 200 page books would be 5 page pamphlets about being overly dramatic.
Twilight comes in a close 2nd