Most of these characters I actually like, just, if I had to pick one.

IV: everyone who "dies"
V: Lenna, just not as interesting as the rest of the cast, but maybe that's necessary in their little band of cartoons
VI: Locke, he seems the most invested in the Returners at the start but there doesn't seem to be much in his arc regarding that as things go on? But it's been awhile, I might not remember enough to give him credit.
VII: Vincent. You realize he's kind of a creep when you get into the backstory of Lucrecia.
VIII: Card Queen just go where I want you to be ok!!!
IX: Amarant. Boing generic tough guy.
X: Yuna. The days before the English localization had its own lip animations made for a lot of weirdness in trying to voice the English lines to match the animation. Yuna really got the worst of it.
XII: Fran has the worst stats
XIII: NORA crew, Snow gets a pass 'cause he's always getting punched in the face over it at least.
XIV: Lahabrea. The whole ascian thing was a big boring drag, which especially sucked 'cause they're the main series antagonist. Shadowbringers really turned that around and has me excited about them but Lahabrea's arc is long over at this point. Not to mention you can't skip the Praetorium cutscenes so you're always seeing him and the weakest point of FFXIV's writing almost every day that you log in.

FFVII: Crisis Core: Genesis. We didn't need another antagonist at all, especially not this guy. All characters original to Crisis Core are detrimental to the continuity but Genesis especially.
FFX-2: Nooj, get outta here with those weird pajamas