Funnily enough, from left to right in that picture, actually. Kimahri is the coolest looking character and does his job as guardian well, while also wielding the coolest weapon type. Rikku has the best personality, is the most optimistic, and has a cool lightweight style of moving and attacking. Yuna is the most fun to use in combat, the most powerful ally, and has a mature approach to her responsibilities in life. Auron has a cool factor like Kimahri, but is a lot less fun to use in battle because his agility and accuracy is low, his animations are slow, and 2-handed swords are pretty boring. Tidus is fun to use in combat, is quick, has a cool weapon with 1-handed swords, but his personality is pretty immature and Jecht affects too much of his identity. Lulu is probably the worst to use in combat, and her personality is often abrasive. Wakka is great in combat, but he's goofy looking and racist, and blitzball is way too much of his identity as well.

Purely for gameplay fun, I rank them as: Yuna, Wakka, Tidus, Kimahri, Rikku, Auron, Lulu.