VI - Everyone who's not a blonde chick with a sword, a twin, or a little girl with a paintbrush.

VII - Barret and Tifa, after Kalm they're almost never in my party again. Not sure why.
Aeris, never found much use for her after Disc 1.
Cait Sith, never found much use for him ever.
Vincent, don't see his appeal.

VII - Zell, probably the same reason I don't use Tifa.
Irvine, I guess I just don't like gun users.

IX - Garnet, Eiko is just better.
Steiner, kinda iffy. I like him and Doomsday Sword is awesome, but he's hardly in my team for some reason or another.

X - Tidus, for some reason when given the choice I kick main characters out.
Lulu, dolls moved to slow. Annoying limit break, Lightning Dodge.
Yuna, never summoned all that much.
Kimarie, never Blue Magic'D all that much.

XII - Everyone not Vaan, Panelo, or Fran.

I have a bad habit of playing favorites. I usually go through the instruction manual pointing out the characters I want on my final team based solely on looks. And it usually works, especially on these games where the characters are customizable enough to make up for any weaknesses you have by leaving people out.