The 'Divergent' series might be considered among the worst by certain critics. The world - building, while initially interesting, has a lot of plot holes. For example, the way the factions are set up and how they interact doesn't always make logical sense. Also, the character development in later books seems rushed.
Some might say that 'The Mortal Instruments' series is not great. There are so many characters that it becomes hard to keep track of them all. The plot gets convoluted as the series progresses, with new twists and turns that seem more forced than natural, and the love triangle aspect feels a bit overdone.
One of the worst could be 'Twilight' for some people. The characters are often seen as one - dimensional and the relationship between Bella and Edward can be considered overly obsessive. It also promotes some rather unrealistic ideas about love and relationships.