This is a challenging topic, because I have fairly eclectic tastes, and because I thought it would be boring to list what seem to me to be the obvious choices—I have no inclination to pick up Fifty Shades of Grey, for instance, and I assume I never will. So it came down to trying to find books that I will never read for reasons other than their utter lack of literary merit. Here are three series that I have avoided:
The Redwall series, by Brian Jacques. I missed the window of opportunity for this one, as I didn’t know about it until I was much past the age of being at all interested in animal fantasy.
A Song of Ice and Fire, by George R. R. Martin. This may not be entirely fair, as I did try to read the first book of the series (A Game of Thrones). Though I’ve been told repeatedly that the series gets much better as it goes on, I honestly did not find anything to distinguish this from every other medieval fantasy I’ve read (and there have been quite a few)—at least not in ways significant or compelling enough to hold my interest.
The sequels to Across the Nightingale Floor, by Lian Hearn. Though I am aware that Nightingale Floor is only the first book in (I think) a five-book cycle, I have always thought of it as a standalone. The story seems perfectly complete and any sequels extraneous. Also (spoiler warning) my favorite character doesn’t survive this one.