For reference:
Front bumper, should have integrated or attached radiator protection.
Dependent on vehicle, some skids may cover more parts than others.
Engine/oilpan: usually, not always, same skid
Transmission/Transfer case: sometimes, not usually same skid.
Gas tank. Important, usually overlooked. Plastic tanks these days can be field repaired as well as steel could back in the day.
Rear bumper: I'm in a pickup. Eventually, when I design/order a rear, it will have wraparounds for the rear bedsides. Also integrated 2" hitch receiver, because my current one kills my departure angle.