To optimize the thermal conductivity of high-density polyethylene, 15 hybrid filler composites containing aluminum oxide (up to 45 m%), graphite (up to 12.5 m%), expanded graphite (up to 12.5 m%), and carbon nanotubes (1 m%), have been designed, using response surface methodology (RSM), taking into account density and melt viscosity variations. It is shown that the addition of filler(s) increases both the melt viscosity (up to ca. 270%) and the thermal conductivity (up to ca. 1000%). Hence, a further increase of TC can be hampered by a too high viscosity to enable processing. RSM indicates that all fillers have a significant effect on the thermal conductivity and synergistic effects can be achieved. The regression model obtained by applying RSM can adequately predict the thermal conductivity of composites of various compositions, as confirmed based on three validation experiments. The experimental density of the cube-shaped specimens is substantially lower than the theoretical density calculated by the linear mixing rule, mainly for the composites with high filler contents. The morphology of the composites, as studied by scanning electron microscopy (SEM), highlighted a good dispersion quality and random orientation of the fillers in the test specimens but also revealed air inclusions in the composites, explaining these density results.