VI: Cyan, Strago, Gau, Relm, Umaro, and Gogo, out of a combination for me not particularly caring about them as characters, and that they weren't as useful as my main team members. I mainly used Terra, Locke, Celes, Setzer, and the Figaro Bros., with the occasional Shadow (because he's cool) and Mog to ward off random encounters.
VII: Cait Sith because he's the worst character ever.
VIII: I don't think I ever once used Selphie unless I was required to (like at the missile base). As a character, I honestly have no opinion on her. I don't particularly like her, but I don't hate her either. So she fell behind.
IX: Quina and Eiko. Quina because (s)he fell behind the others whom I simply liked more, and Eiko because I just can't stand her as a character. Plus, having a second white mage/summoner? I know it was important to the plot and all, but from a gameplay perspective it was pretty redundant.
X: Kimahri. Poor guy, he's the one who tries to do everything without being particularly strong in the same way as a specialist, and in doing so fails to find his niche where he can actually be useful. As a physical attacker, he was outclassed by Auron and Wakka, and as a mage Lulu and Yuna were far better. Combine that with the fact that X freely allows characters to be switched in and out in the middle of battle, and I never found any use for him. :/