Tanks:

DRK > PLD > WAR

DRK is a lot deeper then the other 2. WAR is easily the most shallow from a rotation standpoint imo. It's very straightforward as far as CDs are concerned as well.

Healers:

AST > SCH > WHM

AST because it's so fucking clunky-feeling. WHM because it's very straightforward in most all areas.

DPS:

Melee: NIN = DRG > MNK > SAM

NIN has a ton of pre-planning and micro. DRG has about the same as NIN. It's really close to me. MNK is straightforward, but not as straightforward as SAM. Both need foresight to optimize and are surface-simple but ultimately deep.

Ranged: MCH > BLM > SMN > BRD > RDM

MCH has a ton of stuff it needs to track to optimize, more so then all other ranged. BLM is a surface-simple class, but gets deep and requires a lot more deep class knowledge to get good with, thus ranking it this high. SMN is like BLM but with "minigames" and more movement. BRD is super simple once you work out song rotations and when you learn to push buttons with periods of choices that allow for optimizations. RDM... Do I really need to say anything?