Tidus vs. Yuna vs. Auron vs. Wakka vs. Lulu vs Kimahri vs. Rikku
Tidus has an annoying requirement, but does 1 million damage.
Yuna is easy weapon and has One MP cost, plus summoning.
Auron has First Strike and Full Break, but no multiple hits, but his attack animation is the second fastest, saves you .01 of a second
Wakka does 2 million damage. He doesn't even have to move.
Lulu CAN do 1 million, but you would ruin your hand, effectively not letting you repeat such an endeavor. However, she gets the FEOK and One MP cost with Booster. Unfortunately, the FEOK doesn't pwn your soul. But her magic is long as HELL.
Kimahri can't get past 99999, and Mighty Guard < Ribbon, but ends up with the least compromised stats.
Rikku has Mix, fastest attack animation, and her weapon is easy to obtain.
If you get 3 "2xhits" within 5 seconds for Wakka, then you can have attack that hits 20 times. Easily the most powerful overdrive, and the position of the bars on the slots are easily memorized.
99999 * 20, and the Overdrive does damage equal to a normal attack, which means, barring certain monsters's Celestial Armors, (but his overdrive ignores normal armor), it will do near 2 million damage. Entrust twice, and you can kill Nemesis in about 4 tries, plus a single attack. Or use Aurochs reels, which have the instant kill, and also, the flashing areas are also able to be memorized. Tidus does 9/10, depending on the Blitzball, which is close to 1 million.
It is nearly impossible with a PS2 controller to finish Lulu's overdrive to full, which at that point, it STILL does not do full damage. Small thing I've noticed (though I try not to be racist), is that white and black people tend to rotate with the thumb pressed directly on the stick. Asians rotate with the palm. The palm is faster, and has a less chance of ruining/breaking the controller. You would have to rotate 3 times to get one spot for Ultima, and you only get what, 5 seconds? Even if you rotate even 4 times per second, you would only get 6 castings of Ultima, plus/minus personal luck. The only way to get a full cast, is to have Lulu's magic at 255, and rotate like hell. and nuke the FIRE spell. NOT FIRE2 or FIRE3, FIRE. Even then, you will only get 9 castings (12 is the maximum, I think, but there should be a bonus if you get all of them).
The alternative is to use the N64 controller, which can rotate more times than a PS2 controller ever could, with less chance of breakage.
9999 * 9 does NOT surpass 6 digit damage. Animations for spells are also far more annoying than a really fast character hopping around and killing stuff, with the near guarantee of 99999 damage.
Auron and Kimahri are useless, because their main Overdrive doesn't really get past one hit (Tornado does around 2 in Int, Nova still only maxes out at 99999). But Auron has First Strike, and in the American version, he is the only one with Full Break, overdrive only. If it were the Int version, I would be using Rikku instead.
Rikku and Yuna have Mix and Summon, respectively, and Yuna evolves in a markswoman. End of discussion.
C-huck N-orris has Roundhouse kick. But he crashes the game.
C-huck N-orris > Wakka > Rikku > Yuna > Tidus > Auron > Lulu > Kimahri