i'm still experimenting right now (i started a game to master Yuffie and Vincent's limits, another game to master Barret and Red XIII's limits, and right now i'm working on one to master Tifa and Cid's. I've already done Cait Sith in a previous game that i 'accidently' saved over).

but so far, with my little 'experiments', i'd say that, apart from Cloud's rigged Omnislash, Barret's Catastrophe rules, followed closely by Vincent's Chaos. Red XIII and Yuffie tie cuz Cosmo Memory and All Creation both do 9999 damage once.