In my opinion, motor home roof materials rated best to least.
#1 - Seamless body & roof combination, 100% gel-coat, very thick fiberglass
#2 - Gel-coat very thick fiberglass roof "hat" (I have not seen used in many years)
#3 - Thin Fiberglass sheet stock, rolled over the sides of the rig
#4 - TPO sheet stock, a plastic material
#5 - Rubber sheet stock
Also important is a crowned roof. A flat roof sags over time. You want to prevent water puddles which generally form around the heavy a/c unit. Puddled water also freezes which can be hard on the roofing material.
Rubber & TPO roofs could be more favorable but they are most commonly used in flat-roof designs.
A gel-coat thick fiberglass roof would best to survive a very bad hail storm.