To put it in the simplest terms from my experience from playing MMOs, usually the weakest defensively, will be the strongest offensively. By that definition, Black Mage will be the highest DPS or DD (however you say it) in the game. But like others have said, it very much depends on how you play the class/job, equipment, and also the people in your party as some classes can give buffs to certain classes, but not to others. It's about finding that right mix for optimum damage.