What is Fortunato's biggest weakness that causes his untimely death?
I think that I would argue that it is his pride both in his ability to know his wines and in his superiority over Luchresi. All Montressor needs to do is keep Fortunado focused on the wine - that rare cask of Amontillado. He is luring him to his death with his weakness for rare, fine wines and with the manipulation of the threat to allow Luchresi to try the wine. No matter how sick he is, Foirtunado cannot let that happen because that would represent an affront to his pride. It is his pride, too, that keeps him from realizing that there is an ulterior motive at play. He does not see that Montressor does not like him because he does not want to see it, He only wants to see himself as he sees himself - arrogant and proud and better than anyone else.
Fortunato's biggest weakness is one of three things -- you picke which.
My first choice for it is that he is vain about his ability to know good wine. He really wants to go check out Montresor's new wine and see if it's really amontillado.
Second, he hates Luchresi. All that Montresor has to do is keep mentioning Luchresi and Fortunato is right there.
Finally, I think he must not be all that observant. All this time, Montresor has been planning to kill him and he has no clue that Montresor doesn't like him? Sounds like a weakness to me.
So you pick which you think is the biggest -- they all seem important to me.