People base optimal materia melds on stat tiers these days. So your best bet would be to look up a guide for drg or nin and see what they have to say. For just "preferred" materia you are looking at high crit or high skill speed for Dragoon (to change your GCD timer so that you can double full thrust combo) or high crit and low sks for nin. And you would then prioritize dh > det. If you are just looking for general melds without really caring about optimization