There is some debate about Montresor's being the protagonist since he is

1. an unreliable narrator who alludes to "a thousand injuries"; however,these unnamed injuries may only be such because of Montresor's point of view.
2. vindicative in his punishment, which far exceeds what may have been done to Montresor.
3. a murderer.

Suffering at the hands of Montresor, Fortunato may well be the protagonist.

I have to agree with the previous posts. Montressor is the protagonist, whether the reader agrees with his motives or state of mind or not. Fortunato is his enemy, effectively making him the antagonist.

I agree that we often look for the good guy and label him/her the protagonist.  In truth, the protagonist is the main character.  Some main character are not good guys.  The Cask of Amontillado is a perfect example.  We tend to think of the antagonist as the bad guy, but again that's not really accurate.  The antagonist is nothing more than an obstacle for the protagonist.  This character is placed directly in the path of the protagonist.  The protagonist must then overcome the obstacles offered by this character.  Usually, this character reveals the qualities of the hero, but, in this case, the antagonist reveals the true horror of the protagonist's character.  

Montressor is, indeed, the character who does every major action in the story: he sends the servants away, setting up the scenario for an empty house; he lures Fortunato to his cellar; he walls Fortunato in; and he is thrilled to have kept this secret for more than fifty years. No other character in the story does much of anything except react to Montressor's actions. This is a perfect example of a story in which the protagonist cannot be defined as "the good guy," which is  how so many people define it, and the antagonist cannot be defined as "the bad guy." 

For me, the protagonist is Montresor.  We see the action through his eyes and we, strangely enough, sympathize with him even as he is doing this horrible thing to Fortunato.  I do not think there is any other choice for a protagonist.