Yiazmat was incredibly long, but not that difficult.

Chac was definitely difficult, but I wouldn't say the hardest ever. Same goes with Paragon/Trema.

Vercingetorix, with the right strategy, can be handled, but is definitely intimidating.

Omega Mk. XII, was tricky, but I found Zodiark harder. Don't know why.

Long Gui is not a superboss. A very deathly annoying enemy yes, but a superboss? Not for me.

Gilgamesh and Valfodr aren't really the hardest. They are damn annoying though.

For me, Penance was pure torture because I had to get Wakka's reels and his weapon. Damn you Blitzball!!!!!!!!!!