Proper R-Value/insulation type for between living space and attic area above garage?
For purposes of insulation, a wall common to conditioned and unconditioned spaces should be treated as any exterior wall. Modern standards would be R-19 fiberglass batts (or R-13 for older 2x4 walls), or something modestly above that with spray foam.  

Assuming a proper firewall (5/8" type X gypsum panels, joined over framing members), no special material is needed for the insulation.