The climax of The Unpleasantness at the Bellona Club - Peter meeting Ann Dorland, George Fentiman's psychotic episode, and Dr Penberthy's suicide - is an astonishing bit of writing, with the façades the characters have shown through the story falling away, beautifully worded and with some fine comic touches.
Peter's "The Reason You Suck" Speech to Mary's fiancee, Goyles, in Clouds of Witness, where he blasts him for his cowardice when he stumbled over the body, panicked, and ran for it, leaving Mary to discover it (and possibly run into the murderer), then forcing them to waste half the book trying to track him down. Then the whole thing is wound up with Mary giving him back his ring, saying "I didn't mind you being a murderer, but I do mind your being such an ass!"
It's incredibly evil, but you must admit the crime in Whose Body is also remarkably clever: murder one man, then hide his body by substituting him for a dissection subject, which you then disguise and dump in a random house. Since no crime has been committed involving that particular body, it can't implicate you, while the other one has been dismembered and scientifically disfigured by eager medical students before being dutifully buried in a pauper's field where no one will look at it twice.
Lady Dormer's backstory in The Unpleasantness at the Bellona Club. She told her parents where they could stick their Arranged Marriage, married the man she wanted, and followed that up by running away with him — which involved climbing out of an upstairs window and down a tree in her elaborate crinoline. And to top it all off, she ended up by far the richest of her whole family.