IV - Non DS Version? The Evil Wall. smurf that thing. DS Version? I don't remember, but damn they were all harder.
V - I remember Atmos Kicking my ass for some reason.
VI - That damn Air ship thing that attacks you when you first go to Floating Continent always gave me a hard time in my youth, closely followed by the first Ultros fight when I slacked and found too low a level Banon can possibly be one shot by him.

VII - Throw me in with Carry Armor here.

IX - The Wrong party selection can leave Ark pretty damn annoying to take down, especially if you want to steal all the trout off him and get unlucky.

X - That one fight on top of the airship when you are trying to get Yuna back. And this was only because I got so use to just spamming Eidolons on the boss's I literally forgot hat I could do with half my party members. XD

XIII - Snows Eidolon fight was smurfing lame.

Tactics - the Weigraf Fight when he turns into the monster immediately after always ruined me.