Demon Walls make me tremble in fear...
FFIV: Don't remember fighting it, but I'm sure it was hard

FFVII: Beat it with Cait Sith's EndGame limit (instant death!) then accidentally pulled out the power cord before I saved T_T

FFXII: First one is pure evil, and required me to get a second quickening just to stand a chance, and it still took forever to beat. That sword you get after is pretty decent though.
The second one's a pushover. Seriously.