There are books I started and did not finish that I do not remember. However, there a few that I finished but hated. The worst was:

Reverie - this was a lgbt book club thing in Libby. The protagonist was a whiny incapable teen that never redeemed themself. I kept thinking it would get better and it never did. Things resolved because magic, so poor/lazy writing.

I tend to quit books if I don't find them very good. One I did finish that I fucking hated was The Girl on the Train. All of the characters were fucking insufferable.

The Historian by Elizabeth Kostova

The Pearl by John Steinbeck. Its technically a novella but still. Hated it.

Tom Sawyer. I don't think i'd hate it as much if I read it today, but having to read it in middle school was a bitch

Worst book I've quit is Seveneves by Neal Stephenson. What a horrible book!

Worst I've finished is Grapes of Wrath by John Steinbeck, immediately followed by Of Mice and Men by John Steinbeck. I'll throw in a special mention for The Scarlet Letter and The Great Gatsby. All terrible books that I finished only because they were required reading in school.

The Rings Of Saturn

Was chosen by my Community College English professor and it was the most mind numbing thing I've ever had to read. It was translated from German, so there are multi-page, run-on sentences that haunt me till this day.