I'd say SCH followed by Ninja or SMN followed by BLM

Dragoons and Monks have a static rotation, Bard, whilst difficult due to not having a rotation, is not mechanically difficult, Warriors and Paladins I can't speak for due to not playing either, however from what people have told me, they are mechanically very simple, White Mages are straight up healers with no gimmicks.

Black Mages don't have a strict rotation, but mechanically are still somewhat simple (fire uses mana, ice regains it, switch between the two correctly to win)

Ninja's seem to be complex due to the Mudras, and whilst I don't have experience with the class, they are VERY willing to let you know about how much of a pain they are

SMN at its most basic is simple, however, its complexity comes from how much you need to be aware of at once, three DoTs, a self buff that causes a DoT in an area, your pet, your pets skills and buffs, your aetherflow, your mana, you get put on rez duty to preserve healer mana, and the difficulty is in how far ahead you need to think to juggle all of them whilst also being aware of boss mechanics.

Scholar is even more complex than that, they have the pet and the aetherflow, but their aetherflow uses are even more difficult to plan for due to the fact that a summoner only ever needs to use fester or bane depending on how many enemies there are, whilst a scholar may need those lustrates or sacred soils later, in addition, as a healer they need to focus on the party as well as the boss, AND they are expected to DPS when possible, so keeping track of cleric's stance is important and adds the DoTs back into the equation. plus, they need to know the fight due to their focus on preventing damage as much as healing.