I suppose either Ayn Rand's The Fountainhead or John Norman's series about the world of Gor. For the same reason, really - repetitive conversations and bits where the prose and dialogue all becomes an extremely obvious mouthpiece for the author's ideals about what the world or people's ideology should be like, and it's done at the expense of the story.

This isn't to say that fiction shouldn't impart a message or ideals, it's just so ridiculously hamfisted. Like everyone (including the prose!) stops to turn to the reader in order to declare "Hello, I just wanted to let you know, this is the theme again." It's very off-putting.

Also, John Norman required an editor, because there were lengthy bits where he would go off on tangents that were completely bananas (an uninterrupted page and a half or so of the prose describing the existence and operation of salt quarries in depth). Someone probably should have tried to stop him.