Well, I voted total war as shown, but you could +1 the vote on Stellaris as well since I missed adding a vote for that. Half vote for Fallout 4 atm, maybe later full vote.
Now I haven't played Stellaris, but I'm definitely a fan of Space Sims for a many years. Stellaris is a 4x hybrid, but still looks ok. But as far as space sims go, for single-player, by far
X3:Terran Conflict or
X3:Albion Prelude (X3:AP is an expansion) are absolute kings of the space sim genre. Just as much as Skyrim is to the fantasy RPG genre imo. 8 and 5yrs old respectively, but graphics still look great (dx9), game play well polished and stable (very deep sims and still receives maintenance patches hah just today in fact), lots of excellent mods for customization.
As for total war, hard to ever go wrong there.