WoW these days is pretty dumb.

Last time I played it I basically just mashed my hand on keys 1-5 and did top DPS. They've been simplifying everything for so long that "Rotations" are simply a priority based on CD's. It's actually possible to set up 1 button macros to do the rotations for you.

There was occasionally a spec that was difficult to play (Or literally impossible to play perfectly for humans...) usually something like Feral Druid but they were few and far between.

XIV is pretty simple for most jobs. Especially melee jobs which are focused around just doing a 123 combo or two (Though MNK can be a bit trickier than most due to needing to get the positionals right for maximum DPS as well as utilizing Tornado Kick as a DPS gain)

Tera is a similar kind of priority rotation as WoW, but with more variance, more skills to use and less ability to just macro everything to 1 button.

I'd probably put Tera as the most difficult rotations, but they're not particularly hard.

Then maybe XIV? Hard to say really, since rotations in this game are pretty easy. Like the only "Difficulty" comes from clunkiness, such as NIN with Mudra lag, or MCH with heavy ping reliance due to the timings for their burst window, BLU with it's oGCD skills having cast times so they clip GCD's horribly...

With WoW being easiest because if anything actually makes you require a brain in that game it gets nerfed so you don't realize you're playing a pile of crap...