In Poe's "Cask of Amontillado," Montressor's idea of perfect revenge is confessing to us that he murdered Fortunato over fifty years ago and knowing that he has gotten away with it and there's nothing anyone can do to punish him for it. In short, the idea worked as the perfect muder and the perfect murder story.
Not only was Montressor's plan brilliant, luring Fortunato to his catacombs using a bottle of amontillado as a red herring, but his execution of the plan shows bravado. He foreshadows his vengeful intentions to Fortunato several times on the passage down to the niche using the trowel, the coat of arms, and much verbal irony. But Fortunato is blinded and drunk by greed and wine, playing right into Montressor's trap.
Montressor's plan to bury Fortunato alive in an underground vault also shows much forethought. No one would think to look for him there; much less, there could be little proof of a murder, since Fortunato's body is walled up and mixed with other corpses' bones.
The real brilliance of the revenge, though, comes in being able to retell it over fifty years after. Either Montressor is a brilliant mastermind or a lunatic who invented the whole story. Either way, as an old man, it takes a masterful mind to remember/invent such a horror story knowing that he has impunity.