How do you define "main job"? Is it the job you play the most? The job you enjoy the most? The job you play the best? The job with highest item level? These can all be different for one player and one character.

I play WHM in coil 2 and it's my highest item level job. SCH is my second highest item level job but I almost never play it. Outside of coil 2, what consists most of my play time, I usually go as BRD. Lately I've used PLD every chance I get. I guess I would still consider WHM my "main job", but for me it sounds like the job I do everything with, or spend most time on, and that couldn't be farther from the thruth.