As has been said, any character in FFX can be developed so as to deal 99,999 in a single hit.

The amount of damage you can dish out in FFX is unreal.

I think that FF7 has the most damage potential of all the PS1 Final Fantasys. I was able to deal out enough damage to kill Emerald Weapon in 4 turns (a turn equating to an action by one party member), or under two full rounds, and it's assumed he has around 1 million HP.

Each character has two mastered Magic Plus materia equipped, boosting all three party members' Magic stats to 255, and a Mime materia, with one character having W-Summon. Each character is at level 99, but that's not important.

W-Summom Knights of the Round, and mime it twice in the first round = (9,999 x 13 x 2 x 3) = (129,987 x 6) = 779,992 damage in one round, which is astounding for FF7. Mime again, and Emerald is toast.

Ruby likes to counter KOR, though, which is one of the reasons he's more difficult.