Probably Land of the Painted Caves, the final book in the Earth's Children series. I started reading the series when I was waaay too young (smut when I was 12 probably explains a lot about me...) but still followed along with them as I grew older. The author did a fantastic job world building pre-history life of the proposed (debatably wrong) idea of different groups of humans as well as showcasing the development of the relationship between the two leads as they travel across pre-history Europe. Then the author waited almost 10 years to write the Land of the Painted Caves and it doesn't even remotely capture the charm of the previous novels. It's basically a slogfest through a whole bunch of caves (accurate title is accurate) with illogical drama that basically treats all the relationship growth of the main pair to have never happened at all. It's pretty clear that it's a case of 'did not read my own material' as she ignores everything that came before beyond the names of the characters.
Honorable mention for the Twilight series. Not because it's poorly written but because there is actual potential with the world building. Behind all the glittery vampires and the pedo-wolves, the author did come up with an interesting take on Skin-walkers and vampires. The idea that the existence of the werewolves was all in response to the arrival of vampires was kind of neat. The more vampires that showed up in the town, the more kids with the werewolf bloodline would transform. Unfortunately, it's such a small part (hell, even the characters realizing this is a bit of a throwaway line in the last book) that everything bad overshadows it all.
