FFII - Leon or Minwu. Seriously. Minwu would own everyone.
FFIV - I second placing Edward representing FFIV. We already have a Summoner so Rydia is out. Not so sure about Rosa. Having a musician would be pretty sweet, and who better than that spoony bard? xD
FFV - I would say either Galuf or Faris. Galuf would make more sense if placed in a previous war since....well, we all know.
FFVII - I'm not its biggest fan, but I've grown to like Vincent.
FFVIII - After playing the game twice, mind if I say.....ZELL?! Oh, yeah! I can already imagine the Duel EX Burst.
FFIX - Vivi, Amarant, or Steiner. Another Black mage wouldn't hurt and neither would another Monk (Dissidia already has two). But hearing Steiner clank with each step would probably own both choice by a landslide xD
FFX - I would say Wakka but Tidus already throws blitzballs so Auron would be the best and most logical choice, given his connection to the FFX cast in Dissidia.

Still haven't read up on anything after FFX so I'll stop here.