Who is the antagonist of this story?

An antagonist is a character (or group of characters) that opposes the protagonist either directly or indirectly. Edgar Allan Poe penned the story "The Cask of Amontillado" in 1846. The protagonist in this story is Montresor. The story is told from his point of view, as he describes his experience with his enemy, Fortunato. The story is a description of Montresor's plot of revenge, and his execution of this plot. Fortunato is the antagonist of this story. 
Montresor never gives details about what Fortunato did to antagonize him, but he does divulge that Fortunato has insulted him. In the opening lines of the story, Montresor explains: 

"The thousand injuries of Fortunato I had borne as best I 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."

Montresor's family motto is "No one insults with impunity," meaning that no one will be able to insult them and get away unpunished. Although we don't know the specific nature of the insult, we know that Montresor has been offended and is not going to put up with it. He has bided his time, weaving an intricate plot of revenge in order to punish Fortunato for the wrongs he has committed against him.