What are the Most Controversial SF/F Novels of the Past & Present?

From SF Signal (easily my favorite scifi blog on the net):

There’s plenty of recent controversy in the science fiction field, most of it having nothing to do with books themselves. So let’s put the controversy limelight back where it belongs. We asked a panel of esteemed guests the following question:

Q: Which science fiction or fantasy novels, past and present, do you consider to be the most controversial? Why?

Read their answers below…and tell us which ones you think are controversial.