It would help to know what you have already leveled and what type of roles you usually like to play. But gong off of what you mentioned, RDM has a pretty easy rotation, and nice utility with VerCure and Ver Raise.

BLM, IMO has one of the most complete feeling rotations, beautiful spell animations, and big damage numbers (if that is your thing). They have no group utility, and difficulty is not in the rotation, but knowing the fights and where to place yourself.

DRG is a pretty busy rotation, but very satisfying to pull off. They have a good mixture of self and group buffs. The only complaint most people have with them is their animation lock due to jumps, which can lead to you dying if timed poorly.

Hope this helps. Enjoy which ever you decide to level.