I neglect Vincent and Barret in every playthrough. I try to use them but then I forget and go back to Nanaki Cid Tifa and Yuffie. I don't count Aeris as not being used because she is an amazing healer despite you only have her for half the game. Nanai is a doggy, Yuffie is a Klepto and Tifa beats everyone up. Watching her pick up Safer Sephiroth and slamming him into the ground never gets old

Cait Sith I used on my first playthrough because he was cool, but he is a spy and for that he never sees battle >:/