Weapons generally deal x amount of elemental damage, but they can also deal x amount of raw damage.

I have a diablo 2 background and I'm not quite sure of the mechanics of d3.

Is elemental damage incorporated in crit hits?

Is any form of damage optimal? IE holy damage vs lightning vs simple Damage

Yes.
Most people prefer Damage rolls over elemental dmg rolls. But theres no element that's better than the other.

But some other items, with certain combination might proc better.
As in a wpn with Lightning dmg roll on it, will proc the passive on a ring, that works with lightning dmg, to stun enemies.