Hunter, in Darkness
The plot--you're exploring a cave, hunting a mysterious beast called a Wumpus--is derived from the ancient minimalist BASIC game Hunt the Wumpus, but the similarity ends there. This is arguably the most richly described cave in the history of IF, and your experience of it is thoroughly and harrowingly described. In fact, the cave is as much your adversary as the Wumpus itself, and it takes just as great a toll on you. The puzzles are fairly conventional (though the last one is rather elegant), and they include a maze that isn't really a maze. But the story is brilliantly executed--the plot branches and rejoins so seamlessly that you're unlikely to notice that there are multiple ways through the game--and the writing is terrific; Plotkin is adept at using all the senses. Hunter... breathes new life into a very tired genre, no small feat.