Just like the old resist system, you want to mitigate the damage you are receiving. Investing heavily in xray resist at the cost of other damage types will lead you to have a setup with good xray resist but poor resists otherwise.

Generally speaking pierce and energy are the primary damage types, with fire, poison, xray, and force being secondary. This isn't entirely true when you bring monks and/or flamethrowers into the picture but it's not a bad rule of thumb.

One thing that IS new is that Psi Resist now actually mitigates all PSI-based damage. Back in our time, it only resisted Psi attack 1 and Psi attack 2 as those were the only attacks that did 'Psi' damage.