I was going to say replaying but I feel like that limits the question to games like Prey or Fallout New Vegas that have endings and games like The Sims 2 or Cities Skylines where you can play indefinitely end up excluded.
I’m not really referring to games like League of Legends where you’re coming back every month. More so games where you stop playing for an extended period of time.
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.
You’ve encouraged me to try that game out. Thanks!