As much as I'd love to say Monk is the hardest (aka most skill intensive) class, sadly I'd have to give that to DRG

DRG share a LOT of similarities with how monk works, however though both have flank and rear rotations with a DRG if you do a move in the wrong spot (or if the mob turns before your move finishes) you will lose the effect you're doing the move for in the first place where monk will just lose damage and still maintain whatever buff/de-buff they were applying. Also DRG has more options of buffs and CDs to throw in during their rotation, where you fit them in can have a large impact on your DPS. They do have a bit less to worry about than monk though, as keeping up GL3 on monk can be difficult (or sometimes impossible so you have to plan your PB to get it back quickly) and have a much easier time AoEing than monks (which again requires good use of your PB).

The casters are both easier than the melees. I personally am not very good with Blm but the rotation isn't hard by any means, I just can't stand the long cast times and never got great and finding those nice windows were you can stand still to get everything you want uninterrupted. Summoner has a lot more options than Blm, but they have a ton of more mobility making them easier to utilize as well. I rather enjoy my smn and don't find it hard to play, but I'd like to say that it is indeed more challenging than blm.

Bard is hands down the easiest dps to play, yet a rather large amount of bard players still manage to be bad at it somehow. Anyone can play bard and do good dps with it, its the song management to help the rest of your party do their jobs better that makes someone stand out as a good bard player.

So from my perspective:
1. Dragoon
2. Monk
3. Summoner
4. Black Mage
5. Bard