From hardest to easiest:
1 - Machinist: have never played this one, but the idea that min maxing this class is hard as **** seems to be pretty unanimous.
2 - BLM: not many buttons, but enochian is a complete nightmare to keep. Requires studying fights very carefully. Losing enochian is a harsh DPS loss. Most static class. Even when mastered, going blind into a fight can make you commit suicidal (eg. Seph EX). Doing flawless DPS with this thing on a fight that requires a lot of movement should net you an achievement. Extremely punishing.
3 - DRG: lots of buttons, BotD timer, random positionals, OGCDs that lock you. The class is extremely overwhelming at first, but it loses all its complexity once you become familiar with the rotation (unlike the BLM which is always more exposed to random mechanics ******* them).
4 - SMN: same issue with DRG. Lots of buttons at first, but once you muscle memory the priority system, the class is extremely mobile and no mechanics can really **** you up (other than festering an immune boss...but that's some nasty bad coincidence). Still, takes some time to learn, because of the awful amount of buttons that requires playing this class optimally and the added mana management difficulty this class has. You aren't a flawless SMN if you aren't using Ruin III outside your Dreadwyrm Trance when you have mana surplus.
5 - BRD: Arguably easy, but not as easy as most people think. The class doesn't follow a strict GCD order (except for the opener), it's a complete whack-a-mole OGCD fest, and it can be pretty frenetic at times, when multiple OGCDs come up at the same time and there's very little room to weave them all. Also, must keep track of DOTs, not refreshing them with Iron Jaws is a DPS loss (though not a harsh one).
6 - NIN: Can't say much about this one (level 0 rogue here), but I've heard it has a very flexible rotation, which is easy and forgiving too.
7 - MNK: It's an easy version of the 2.0 MNK. Not only losing GL3 is much harder now due to form shift, even if you lose it during a long downtime, stacking five chakras for a free 330 potency attack + starting with instant GL due to form shift makes this class much more forgiving than it was in 2.0. 7 GCDs, and a pattern that is extremely set in stone. Very easy to become familiar with, and very easy to do extremely well. Once mastered, there's hardly anything that can screw you up, class is mobile and very noob friendly. Missing positionals doesn't punish your DPS much, but landing them is easy enough anyway.