Anyone who hits 9999 damage first gets anyone killed. Except Tidus can have 99,999 HP. Even so, in that case, Cloud has KotR and Omnislash, Squall has Renzokuken and Leonhart, and Zidane has Steal (no sword and no armor on Tidus = no Break Max HP and no Break Damage Limit).

My pick: Fully-Equipped Onion Kid.