>>533844970
It doesn't matter until you're in ranked, and then it becomes whatever you're most comfortable on because that'd be your best chance at winning.
In casual, you're stuck with wildly varying team makeups that determine the game 90-95% of the time.

If you want to pretend you're helpful and never contribute to crystal progress; MCH, WHM, SMN, BLM, AST, MNK, NIN are easy picks for you.