SCH is far and away the most complex and demanding job to play at an optimal level. It's unique in the sense that no other job really needs to proactively anticipate major and minor mechanics, maintain strong DPS, and keep people from dying all at once. When you couple that with the fact that the Arcanist class is OP by virtue of having more buttons to press (and thus more options) than any other role in the game, things get daunting.

A close second is likely between WAR and SMN.

DPS in general is hard to truly maximize and compete parse for parse. But, honestly, the top 10 or so players in the world for any given DPS role really have it easy outside of a progression environment where organizing callouts play a much bigger part than in farm content. You're essentially just hammering away at a rotation that you've been doing forever and adapting to the nuances that exist between any given run of a boss fight.

WAR is in the same boat as SCH without being as complex. You're trying to do a few major things all at the same time. Though the job isn't hard to just "play", it is pretty hard to maximize while not jeopardizing the party. You essentially want to maximize time out of Defiance, manage defensive and offensive CDs at the same time, and snap aggro on adds (and tank swaps, for what that's worth).

The melee DPS and BRD are mixed under there, followed by WHM and PLD easily being the most simple.

There really just aren't enough buttons for WHM to actually press for it to be difficult. Once you've learned a fight's script, the only thing left is choosing the structure of your single target and AoE heals and timing them accordingly. MP is a non-factor unless you're just god awful or too many people are dying to avoidable mechanics making an underpowered class like WHM work too hard.

PLD is 1-2-3 and a CD rotation. You can bolster the responsibility of a role like PLD by giving that player additional tasks like call outs and overall group organization during an encounter.