well, i think that for me it all depends on whether or not i actually like them within the context of the storyline, but for me its...
FFVI - locke - I'm sorry, but aside from when something has an item that is actually worth stealing, i just never use him. at all.
FFVII - Cait Sith - Even though he can be a majorly powerful character, but he is such a #@£$ in the storyline, that i just never want to use him.
FFVIII - I use everyone, as the character customisation is just brilliant in this game.
FFIX - I never use Freya. She just bores me.
FFX - I have to say Lulu and Rikku, as i think that Yuna's base magic stat is just better, and you can teach her to cast black magic anyway. and Rikku is just useless.
FFXII - Ashe and Basch. They are both just puny characters, and the voice acting is the very worst of a very bad cast.