FFI - Black Mage - though I can't help but feel that this is influenced by 8-Bit Theatre.
FFII - Minwu
FFIII - I really don't have a preference.
FFIV - I've always rather liked Rubicante. In terms of playable characters, probably Rydia.
FFV - Bartz, if only because he's a bloody pimp.
FFVI - Celes - I actually found this a rather tough decision. I was tempted by Edgar, Kefka, Leo and Cyan amongst others.
FFVII - Cid - Of course!
FFVIII - Edea. I think it's partly because of the character design and partly because she made a badass villain during discs one and two.
FFIX - A really tough decision, as this game has some of my favourite characters in the series. Given that he used to form part of my username, I should probably say Kuja. Too many honourable mentions to mention though!
FFX - In spite of Square's attempts to encourage male gamers towards a particular character, I'd probably have to say Auron.
FFXI - Not played.
FFXII - Balthier, of course!
FFXIII -I've yet to play.