Murder on the Orient Express: A Hercule Poirot Mystery

Reading this, it's obvious why it's such a classic. Poirot is the very archetype of the detective, the setting is unique and fanciful, and the titular murder is accomplished under mysterious and seemingly impossible circumstances. Throughout the story, Poirot deftly uncovers the most subtle of clues to arrive at a fascinating conclusion. If there's any drawback to the story, it's that for a modern audience some of the clues might be too subtle, as they rely on outdated cultural references that most modern readers wouldn't be familiar with. On the whole, that doesn't detract from an enjoyable story, however.