#1: Rock sliders if you don't have at least rubicon stock sliders, but those don't stick out so don't plan on sliding around stuff on them without body damage
#2: Engine/transmission skid... expensive, but really a good idea
The stock transfer case skid is impressive for stock (at least on my 2014, don't know if 2007's were the same)... I'll probably never upgrade it.
Although not technically a skid, relocating stabilizer is critical and cheap.
Others have said evap skid, while important I don't see it as being so high priority compared to others. I bent mine pretty bad and my evap is fine, and even if you damage it the repair bill is pennies compared to body damage (if you care) or engine damage. Also, consider re-locating the evap rather than putting a bigger skid on it, saves weight.