The best 4X combat. Huge variety of units and sides (need DLCs though). Unique game in that it just focuses on combat, no diplomacy (it sux in all games anyway) no spies (also sux in other games), and simple but reasonably good (and different for different races) economy. It focuses at one thing - combat, drives it to perfection and the rest of the game design narrow focused to support that.

Being W40K helps with richness of the fluff, if this is important for you.