Mine is Space Engine personally.
Orbiter. Kind of really the only space-flight simulator out there. Well, KSP might qualify in a charming, simplified way…
Space Engine really is one of the best.
The spaceflight is really detailed and accurate, and there’s lots of effort put into mapping out every planet, star, and system.
My other big favorite is X-Plane. Though it’s technically a flight simulator, there are spacecraft that can be flown into orbit.
Quite fond of Universe Sandbox 2 myself
I havent actually played it in about 2 years so looking forward to booting it up and checking out all the new features
Heres someone having fun with an engineered system. The goal was to see how many planets you could fit in a stars habitable zone while remaining stable and functioning. The player decides to make it, less stable