Dark knight: you get it in the Bevelle underground after solving a quick riddle with elevators but I dont know it off hand, look it up its easily one of the most damaging jobs in the game. Darkness spam.

Alchemist: You get this if you complete all of the missions for the small fella at the Moonflow. I think.. been a while.

Lady luck: I think you get this if you beat shinra at sphere break.

Dark knight and Alchemist are two of the best in my opinion.