they all have thier advantages and disadvantages.

Warrior- basic tank
Monk- Hit pretty hard
White mage- Healer!
Black mage- Black Magic!
Red mage- A little bit of everything.
Thief- What's cooler than a thief?
Paladin- Good tank, heals
Dark knight- GREAT attack
Beastmaster- MADE for soloing
Ranger- Some of the hardest hits in the game!
Bard- Really fun to play as.
Summoner- nothing, as you can't summon bahamut.
Samurai- enhanced TP gain
Ninja- Big array of special abilities.
Dragoon- Wyverns

I say dark knight. with a warrior or samurai subjob.