Busman’s Honeymoon is of the great mysteries of all time. Its first half is a masterpiece of charm and observation, filled with wonderful characters and a delightful romance.
Unfortunately, Sayers decided that she still needed a mystery to wind the spring of her plot, and at the end of a career of baroque mystery (this would be her last Lord Peter) she turned to one of her most baroque crimes. Chandler famously wrote that the criminal who schemed this one had to have God sitting in his lap, and of course he's right. And there are plenty of ugly snippets of borderline anti-Semitism, nasty class innuendo, and I think more than a dash of sexism in what is trying to be early pop feminism.
But still: Harriet, Peter, the people of Paggleham, and The Dowager Duchess.
(Amazon has this paired with Mystic River: what are they thinking? Two very good regional mysteries that share nothing else in common?)