Just as a point of discussion. Maybe we should talk about priority, I realize this will vary depending on where you wheel, but..

I'd rank all the OEM skids and the Belly Guard highest and then the diff guard. Reason being any damage to what those skids protect and you aren't driving home (if its serious enough)
I would put the rear sliders next from personal experience, I have lightly damaged the underside of my rear bumper. So I think this comes before the front skid in my book because the approach angle is better than the departure angle.

I think I would even put the rock rails before the front skid.

The only reason I see for getting the front skid before the others is the look and if you are a "momentum wheeler" for lack of a better term..

All of this of course is my personal opinion from what I've seen and done..