It's a tough choice, but I have to go with The Cask of Amontillado. This tale is interesting because we don't know why Montresor kills Fortunato. Why do you think he did it? I think he was insane. Here's how it starts:
The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult, I vowed revenge. You, who so well know the nature of my soul, will not suppose, however, that I gave utterance to a threat. AT LENGTH I would be avenged; this was a point definitively settled -- but the very definitiveness with which it was resolved precluded the idea of risk. I must not only punish, but punish with impunity. A wrong is unredressed when retribution overtakes its redresser. It is equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong.You can read the entire tale online at Read Print.