I'll have to say Ultima because her field thing is constantly changing, so it's much more unpredictable what will you have restricted at what point. I mean, there's an order, I think, but still. Plus, sure Zodiaek is the hatdest and strongest and whatnot, but Ultima is literally the second one following the game.

Zalera is just a time limit, pretty easy to deal with even before you get 5 Espers.

For Exodus, you can't use items, and I personally don't really use items that frequently to have that be a problem, so I'm fine.

Chaos you can't attack, but can use magick or even better: Telekinesis.

Zeromus is another one that kinda sucks to me, because I rely more on magick than on items in FFXII, but it's still defeated rather easily.

Cuchulain has the damm sap, which is annoying, and the constant hoard of flans too, but it's also overall easy to get done with, if you focus on defeating it instead of slacking.

Of course, how truly easily each is defeated also highly depends of when you go to fight them, but still.

Zeromus when I fighting I feel like i laughing too much, how many KO do I have ? There spawn skeleton but not too strong around 4 or 5 i think slightly annoying but useful for healing thanks Chain. Zeromus always Use gravija too much I got KO one by one and use gambit auto phoenix and remove poison

Zalera is easy, I just wonder what happen time limit is over ?? Or maybe something "danger ability' ??

Ultima is hardest one ( but not very hard from ultima weapon ) He has weakness dark. When ultima has reach half health, she use Magick barrier.This Magick Barrier will rotate every once and a while between causing the following affects such HP and MP drain, no MP, no attack And other.

Chaos is easy too, just using telekinesis and gil toss

When the time limit of Zalera is over, he uses an ability that just sents you to the previous room, so you literally can try again right away.

NG- would be Cuchulian for me

Ultima + 3 Mithunas with Dark Shot, Brave Suit and Bezerker Bracers have taken her down before the second Holyja