What do you think makes the best material for off-road bodies? Aluminum, Steel or Fiberglass?  

I think 'glass, 'cause it's relatively cheap, and if you have a white vehicle to begin with, you don't even have to paint it if it has a white gelcoat. Aluminum is cool 'cause it's light, but it's expensive and a pain to fix.  

Fiberglass. For exactly the reasons you stated, its relativly cheap to fix, and its flexable. If you get hit by a rock or tree branch the fiberglass will be more willing to give than to dent or crack from the hit.