I like
Battlefield Earth, myself -- the book, not the movie which bore only fraudulent resemblance to it.
However, a think all eleven of Hubbard's "Mission Earth" series ought to be among the worst books ever written. If you can make it through the whole thing, you're clearly one extremely sick puppy ... and not in a good way.
I only made it through the first two, and that was because of a school assignment, if I'd realized what toxic waste it was early enough I'd've probably chosen a different author to write that paper on. Truly ghastly stuff.