The crime and detective system from Salem, hands down.
Salem Online had one kill permadeath, so tracking of crime was important to balance. I haven't looked at it in years so maybe its changed, but they used intuitive "scents" and "evidence", produced as interactable exclamation points, whenever someone trespassed, stole from, assaulted, or murdered anyone else.
The severity of the crime determined how long the "evidence" stayed behind, and how long it could be used to track the general direction of the perpetrator. All you would get was an arrow on your UI pointing in the direction of the criminal - they could be seconds or hours away, and your evidence lasted only so long.
If the perp was found, however, they could be attacked with impunity. People behave themselves relatively well unless they really think they can get away with it or fend off the hunters coming for them.
It was simple and pretty well balanced. A system like that could be iterated on for CoE.