Quote Originally Posted by Seraphor View Post
Casters: RDM > BLM > SMN
Ranged: DNC > MCH > BRD
Melee: SAM > DRG > NIN > MNK

This is personal to me, maybe you'd swap a couple around, but I'm pretty sure the easiest of each role is more-or-less consensus.
Casters and Ranged are spot on. Melee seems to be a mixed bag depending on whether you find positionals easy or not. Personally I'd put DRG and MNK as the easier ones with very little you have to keep track of, and SAM with NIN as the two hardest due to how many things you have to keep track of.