The Prison Prep School and Inspector Kenmochi, Murderer arcs are just terrific! I never labeled myself a mystery fan until seeing the first Kindaichi series, then Tantei Gakuen Q (by the same mangaka), and now Kindaichi R.
As good as, and as profound as the best Agatha Christie, if not even better.
Truly inventive stories, my favorite of which have a sculptural/spatial/plastic turn: the Prison Prep School arc in this series and also Tantei Gakuen Q's kamikakushi arc.