Statistic-wise, Lionheart (Squall) deals a lot of 9999 blows (goes so fast I can't count them). I don't know Zidane's Ultimate, but his thingy 9 (don;t know name, second best) is only one slash of 9999. Tidus' Blitz Ace is also affective, and Cloud's... dunno..

I'll go with Squall