I prefer Montressor over Fortunato as the former is committed to his cause. He has carefully planned and exacted his revenge upon his enemy. While murder is never an option, the reader is unaware of Fortunato's offense against Montressor; perhaps it was extreme? While I should feel pity for Fortunato, I do not. He is an unlikeable character due to his naivety.
Well, that's a tough one. There are really only two active characters (though others are mentioned). One's a crazy murderer (Montresor), and one's a murder victim that we only meet through the killer's eyes (Fortunato).
If I have to choose--and this may say more about me than I want to admit--I'd go with Montresor. He's got energy, and he's focused on a goal. Fortunato seems like a passive dupe.