Favourite:
Black Mage: What can I say? I adores Black Mage.
White Mage: I need a healer though I despise White Magic.
Dark Knight: Mage + Fighter. *Fall in Love*

I use:
Gunner: Trigger Happy + Cat Nip = Total Ownage, PWNED!!!
Dark Knight: Black Sky + Champion Belt
Mascot: Moogle Cureja + Moogle Regenja + Moogle Lifeja + Moogle Wallja!!!
Gun Mage (I replace my Dark Knight with this): People found it to be useless but once you got all of the ability, you PWNED them! Supernova + Mortar + Storm Cannon + Mighty Guard + Annihilator = PWNED

I strongly against:
Songstress: What's the use of status effect now? Unless you want MP Mambo.
Thief: Though they're fast, they didn't do good damage unless if you want to steal something for New Game +
Warrior: You already have a Dark Knight, now off with you!!