Well tiering a character based on their usefulness throughout the game is more relevant to a person’s interest. Who cares if Vincent can do all this crazy stuff if he sucks throughout most of the game? If I saw your list and used that to determine my characters to use in the game, I would be massively disappointed in Vincent. And also, Barret can do the same glitch as well through massive materia bonuses and OHKO everything. He’s also the only character who can naturally reach 9999 HP. He should definitely be higher than Cid even if you didn’t know that Barret could accomplish the same glitch, since you forgot to take into account that Angermax is a level 3, and Highwind is a level 4. If you want to argue for JUST potential, then Cait Sith should be high instead of low, since you have the POTENTIAL to always hit Game Over on his slots. Perfect timing can get around the game trying to screw you out of it. Yuffie’s conformer being better than the Ultima Weapon isn’t the major reason for her being better than Cloud in extreme endgame setups, its that Doom of the Living is a level 3 instead of 4. Red XIII should be higher than Tifa, since Stardust Ray hits 10 times, and Tifa’s limits can only go up to 7 hits, and Stardust Ray is a level 2 limit compared to level 4.
So, the list should be something more like:
God:
Vincent
Barret
Top:
Cait Sith
Cid
Middle:
Yuffie
Cloud
Low:
Red XIII
Tifa
Aeris
However, you’ve forgotten one very important thing. At extreme endgame, you should have enough Knights of the Rounds and W-Magics to give to every character. Meaning that only Barret and Vincent still remain better than everyone else, since they remain the only characters able to pull off insta-kills through overflow. Otherwise, everyone else just W-Summons KoTR. So the true list for endgame is:
Top:
Vincent
Barret
Not top:
Everyone else
Ayla has good things going for her besides overpowered 9999 damage. Slurp Kiss is incredibly awesome cheap and effective MT healing throughout the entire game, and Falcon Hit is absurdly powerful.
Tidus IS better than Wakka for most of the game. Its only when you’re trying to tackle super bosses when you really need to use Wakka’s Attack Reels to deal the damage needed before you die. But Yuna beats them all since she can Zanmato everything.