SMN. I couldn't tell why, there is just not one single redeeming aspect, I hate everything about the job.
Since ACN was the only class not already at 50 when ARR was released, I used it for the main story. After that I've only used it to finish some of the job quests, but I have since given up on those too. If I never have to touch it again it'll be too soon. Massive disappointment of a much anticipated class/job.