I have only leveled up 4 jobs to 70 so far but for speed and stuff to do, I would put them from most to least as Bard, Samurai, Monk, Black Mage. Monk has the fastest regular GCD, but Riddle of Fire and lack of oGCDs make it not really very interactive anymore with the removal of their second dot. Bard and Samurai both have lots of oGCDs, Bard has a lot of procs (based on song used) where Sam doesn't (based on resource buildup). BLM is a typical caster and not very fast so you wouldn't like it.