For me it was One Thousand White Women: The Journals of May Dodd, by Jim Fergus. I simply could not get past the disgusting premise of sending the dregs of post-Civil War women out west to marry/pro-create with the Native Americans to assimilate the "savages." I was offended on so many levels.
This is followed closely by The Favored Child, by Philippa Gregory, where she took the incest theme to entirely new and yukky levels.
