My strongest character would be either Tidus or Rikku.

I liked Lulu until Rikku came along and learned all her black magic.

Well, I still like Lulu, just not in battle.

Tidus is just plain strong. I had him learn all of Yuna's white magic after he finished his grid (which I did by like the point of Gagazet) and he rules.

Rikku's mug ability is something I use on everyone. It helps.
And when you have things like petrify s and warrior overdrive mode?
I can deal massive damage with Rikku's overdrive now.

Yes, Yuna has aeons that are uberstrong, but besides her aeons Yuna isn't too helpful.
As a matter of fact, Tidus got Auto-Life before she did.

My worst character is Kimahri. If you ask me, I think he has no point of being in the game. I brought him into boss battles to defend then leave, so he'd get AP and (SPOILER)be strong enough to defeat his brothers. He learned Lulu's black magic.
After he beat them, I stopped using him. He's useless, anyways. I don't like his overdrive, either. I think it's stupid.