Strongest is hard. Depends on how you define it. I mean on one hand you have the raw destructive power the Black Mages and the ability to channel the powers of Elder Primals with SMN but you also have WARs who has heroes remembered for things like tearing the arms of a Behemoth with his bare hands (Mythril Heart must have been a god mode badass).

I don't think there is an easy answer to this.

Weakest currently would probably be MCH which is still in its infancy but it has massive potential and theoretically a person with stronger Aetheric reserves could potentially have some extremely powerful attacks.