Zidane, Eiko, Vivi, and Steiner for me.

I love the thief class, especially in IX. You can keep stealing after you grab an item, so you don't have to reset if you grab the common item.

Eiko, has been pretty much covered. Better spells and her Trance is mean. Dual Cast Holy is just well mean. And I have no idea what people mean when they say Garnet is better to look at. Well I do, but I find Garnet ugly and with a big head.

Vivi, adorable little bad ass. Pewpewpewkapew!

Steiner by him self is pretty decent. But combo'd with Vivi and Doomsday Sword he's the best.