My nomination for worst book I ever read all the way through was
100 Years of Solitude by Gabriel García Márquez.
Here is the blurb about it at Amazon:
Editorial Reviews
Amazon.com
"Many years later, as he faced the firing squad, Colonel Aureliano Buendía was to remember that distant afternoon when his father took him to discover ice."
It is typical of Gabriel García Márquez that it will be many pages before his narrative circles back to the ice, and many chapters before the hero of One Hundred Years of Solitude, Buendía, stands before the firing squad. In between, he recounts such wonders as an entire town struck with insomnia, a woman who ascends to heaven while hanging laundry, and a suicide that defies the laws of physics:
A trickle of blood came out under the door, crossed the living room, went out into the street, continued on in a straight line across the uneven terraces, went down steps and climbed over curbs, passed along the Street of the Turks, turned a corner to the right and another to the left, made a right angle at the Buendía house, went in under the closed door, crossed through the parlor, hugging the walls so as not to stain the rugs, went on to the other living room, made a wide curve to avoid the dining-room table, went along the porch with the begonias, and passed without being seen under Amaranta's chair as she gave an arithmetic lesson to Aureliano José, and went through the pantry and came out in the kitchen, where Úrsula was getting ready to crack thirty-six eggs to make bread.
"Holy Mother of God!" Úrsula shouted.
And listen to this crap. . .
William Kennedy, New York Times Book Review
"One Hundred Years of Solitude is the first piece of literature since the Book of Genesis that should be required reading for the entire human race. It takes up not long after Genesis left off and carries through to the air age, reporting on everything that happened in between with more lucidity, wit, wisdom, and poetry that is expected from 100 years of novelists, let alone one man...Mr. Garcia Marquez has done nothing less than to create in the reader a sense of all that is profound, meaningful, and meaningless in life."
When the book was over I wanted to demand the hours of my life I had wasted reading it back. What a waste of paper. .
~Lyr
I like The World According to Garp. 
Found it blackly funny, and reread it every few years. 
Stunned silence . . . . . The book's a masterpiece! Well, each to their own. 