Mill House Murders

While I won't say that I hated The Decagon House Murders (because it was enjoyable), this book is so much better than that one. I think this is where he really got a feeling for the 'detective' character Shimada Kiyoshi (who isn't really a detective) and using his other characters to actually feel more part of the crime and the story itself.

He also does a better job at using the house gimmick in this book, since that's a key component of the stories in the Bizarre House Murders series. I really like the inclusion of characters just going "Wait, this is a Nakamura Seiji house? Shit." (They don't actually say that, but that's the impression you get when the characters realise what's up.)