The whole cast of IX was really strong, but if I'm being really critical:
Zidane - all around good guy, but he comes on far too strong to Dagger at the start, even molesting her as they get on the cargo ship.
Garnet - dull, and frustratingly naïve (about her mother) in the first half of the game.
Vivi - the awesome.
Kuja - tries too hard.
Eiko - brat.
Freya - seemed interesting at first, but then her story just kinda stopped and she wasn't involved much in the rest of the game.
Steiner - great fun at the beginning, great fighter at the end.
Beatrix - it's so annoying that she can't be defeated. YOU HAVE 3630 HP. I HAVE LIMIT GLOVE!
Cid - meh.
Lani - never liked her.
Amarant - full of himself, and has to be rescued in Ipsen's Castle. It's a good thing that the world waits for you to save it.
Anyone else - how come Quina isn't in the poll? Not that I would vote for him/her. Garland was pretty cool, but not involved in much of the game. Same goes for Mikoto.
So it's between Vivi and Steiner. Impossible choice, I love them both (and they even work together for the Magic Sword ability). I think I'll go for Vivi, as I felt more emotionally involved with him.
EDIT: Steiner has no votes!?