I have all jobs leveled up. While I play some jobs more than others, I still end up playing all of them at least once in a month.

When it comes to preference, I'd say:
Ranged DPSes (BRD>BLM>SMN)
then Healers (WHM>SCH)
then Melee DPS (NIN > DRG> MNK)
Tanks

When 3.0 release I'll main SMN, but will use SCH to queue/level faster

I'll then level up the remain DPS while AST/DRK are still a new thing