Bard is the easiest imo. There are many moments in rotation a blm can optimize that edge it out over bard in difficulty for me, blm can have good timing on use of procs, proper use of transpose between stacks (especially with firestarter, you get about half a second where the game still thinks your in umbral x3, so if you transpose that last moment before your final mp tick and hit your firestarter proc you lose almost no time but gain about 2x the damage on firestarter, small increase, but it adds up when timed right), can mix in blizz1 or using the right thunder for the timing of tick instead of just doing thunder 2 all the time like most blm, etc) all of which require more attention to perform than bard, and of course you have to be standing still to cast, bard can run circles and do near top dps still, just keep your buffs going, dots up and spam away. SMN tops the list in difficulty, and Mnk then drg 2nd and 3rd respectively.