He is definitely not fortunate. However, you could also argue that fortune can apply to any kind of fortune- good or bad. So Fortunado is doomed to die, because Montresor has decided he will. Therefore, he has bad fortune!
I tend to agree with #3. It's true he kills his apparent tormentor; however, he does not fulfill his own definition of revenge. He says:
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.
Whatever the most unfortunate Fortunato has done wrong, Montressor "fails to make himself felt as such" to him. Fortunato dies clueless as to why he has been condemned to death.
Montresor himself would probably say that he gained revenge; he premeditates a crafty and detailed plan to dispatch of Fortunado, and then follows it through with great delight and enjoyment. He even pauses at one point to more fully enjoy the tortured noises from Fortunado, and when Fortunado starts screaming, he screams louder, mocking him. At the end, there is a hint of regret as the narrator states, "My heart grew sick" but he brushes it off, blaming "the dampness of the catacombs". In the beginning it hints at the "thousand injuries" that Fortunado had given Montresor, and that was the reason for his insidious scheme; so, if you feel that holing a man up in the catacombs is the revenge for those injuries, then yes, his revenge was had.
Fortunado's name is ironic (the opposite of what you'd expect) because one would expect that a man names "Fortunado", which brings to mind "Fortunate" would have a better fate than the horrific one that he has to face at the end. Fortune did not smile down on him at all; in fact, it seems his luck, or fortune turns on him in the end.
NO, he does NOT get the revenge he hoped for. He takes time to explain in detail his definition of revenge in the beginning of the story: he must be sure his victim knows WHY he is being punished. Montresor never accomplishes this.
When he says "my heart grew sick"--he means it, and then makes a lame excuse about the dampness. He failed because Fortunato died before Montresor could explain the injustice done to him. Whereas he slowly built the first rows of bricks, enjoying Fortunato's pain, he hastens to finish the last row. It's no fun anymore.
Perhaps Montresor did not mean to kill Fortunato, just to scare him. If Fortunato were to tell others of the event, no one would believe him: he was drunk, no servants witnessed the event (and they would lie about being home because Montresor had given them explicit instructions to stay home), and the story is so fantastic that Fortunato must have made it up. But the plan faied. So, Montresor has NOT gotten the revenge. He did not get to hear Fortunato apologize and beg for his life; he did not get to threaten to brick him up for real if he ever insulted Montresor again.
This is why Montresor says Rest in Peace at the end of the story.