Which Final Fantasy Boss was the hardest (non-optional) up through 13

[Wolf Kanno]

Difficulty is hard to pin on FF bosses, as long as you know either a) what to expect, or b) the special "trick" to beating them most of them are pretty damn easy. I never had issues with most of the bosses mentioned so far with the exception of Garuda but he's a "trick" boss and once you get a full dragoon party he goes down pretty easy. Also, Yunalesca goes down pretty quickly if you walk into the fight with a full party (Aeons included) with full Overdrives.

The other boss that was mentioned that I feel is justified is Cloud of Darkness cause it's a bitch to kill if you are not leveled high enough and brought the wrong classes, notice I said and... Zeromus in the original FFIV is also a rightful pain in the ass cause even though there is a trick to beating him, that's only half the battle and he is a cakewalk in any version that isn't FFIVJ or the Chronicles version.