Back when I was still in my misguided "even bad books are books and therefore sacred" book-finishing stage, the worst book I ever read was The Temporary by Rachel Cusk. I actually threw it across the room in anger at its overwritten pretentiousness once. We had a Golden Retriever puppy at the time who had a tendency to chew books if left alone for more than a few minutes and even she wouldn't touch it.
Since then I have seen the light and realised life is too short to waste on bad books. I have Love in the Time of Cholera to thank for that epiphany. Three times I started it, three times I failed to get to halfway.
I had to study Wuthering Heights for A level and remember hating it but not being able to put my finger on why. Now I can see that all the characters are so unremittingly unlikeable and overwrought.
I could never get away with Dickens; all coincidence and melodrama (although I love Armistead Maupin's Tales of the City which are equally coincidental....go figure).
I enjoyed reading We Need to Talk about Kevin but oh my gods, the tennis book was just dire! I'm sure it's a very accurate portrayal of the self-centred ruthlessness of a certain type of sportsperson but it made for a turgid, depressing book full of characters I hated.
Finally - need to rein myself in here or I could go on all day - while I enjoy some chick-lit, including by some of those writers mentioned upthread such as Jill Mansell and Marian Keyes, there is a very small bonfire in Hell reserved for the work of Wendy Holden. Sub-sub-sub-Jilly Cooper, with stereotypes instead of characters, lazy slapdash writing and a bad pun for the title which she clearly came up with first and then wrote a (staggeringly unfunny) story to fit. I got not one but two of her meisterworks free with different magazines back in the day - that should have been warning enough.