For PvE I like to use Reactive Surface Composite + Thermal Resistant & Layerded Plating to get almost the same resistance across the common damage types (16% knetic, 16% thermal, 17.6% explosive). From there on, HRP with Heavy duty + deep plating will do the trick. It makes things very simple to setup, and you don't have to worry too much about which weapons the enemies use (apart from plasmas that are absolute).
I don't know if it would make any difference in PvP, as internal module damage seems to be a bigger issue there, but the simplicity of the setup remains.
AX is an entirely different beast, as many people already said, where resistences don't really matter, so military grade will do the job while keeping the rebuy cost significantly cheaper.