For regular daily driving load C is good. Just don't go below the weight range for your truck. Check out tirerack.com if you haven't already. Search by vehicle, and it will list what is right for your truck. If modified then search by size you need.
Offroad C and D work well too. You let air out of the tires so you don't want the super stiff sidewalls to prevent the tires from wrapping over a rock.
Load E for full size trucks and pulling/hauling heavy loads. Offroad maybe the desert runners use load E. Hitting rocks at full speed. But then even a BFG BAJA TA is supposed to be crazy stout and is only C.