FF6- I never really liked Gau or Umaro. The only reason to even GO into the Yeti Cave is the Terrato/Midgardsormr Esper. Both are just auto-attackers that pretty much screw you if they're the only ones left alive in a battle. Plus, WHY is Gau even in the story? He doesn't offer anything else except a stupid cut-scene and a guide through the Veldt.
FF7- Hmmm... I'm going to have to go with Barrett. I only use him in the first disc until I can put a team of Cloud/Aeris/Tifa together. Barrett makes a nice barrier if you equip him with Cover materia and a load of HP Plus, otherwise he's not exactly a good choice.
FF8- Hmmm... I'd have to go with Selphie. Slot is just too random to be effective, and I just HATE her whiny little self. "Oh, let's go to Trabia so little miss whiny will shut up!"
FF9- Quina. She can eat enemies to learn Blue Magic, but the Blue never really stays up to par with the enemies you can encounter. And her Trance attack is Cook- just a better chance to learn Blue Magic, with no damage. Useless.
FF10- Wakka. Good grief, he sucks. "Oh, I can throw my blitzball at things and make an unreliable overdrive for a POSSIBLE element attack?" Lulu is slow, but that's why you throw Hastega on everyone and proceed to kick faces.
FF12- Penelo. I like her character, but by the time you get to the point that you can use her in battle, she's already so far behind the other party members that you already have an established team.