I recently read Daisy Darker and I'd have to say that one because I felt like the twist was done so poorly. The fact that well...if you read it you know...., was very heavily hinted at since the start of the book makes it pretty predictable, but there were a few other very tiny things I noticed that would have directly contradicted this twist if I read it correctly.

(Also I read The Couple Next Door a few months ago and honestly barely remember the ending. Or anything else about the book.)