Star Ruler is a 4X/RTS Hybrid on a grand scale. Mod everything, design anything; play what you want, how you want, when you want.
Quite possibly the best space strategy game I've ever played. I could go on for a while about its solid 9/10 gameplay, but I'm going to just skip to what earns it that perfect score: the freedom.
Everything in the galaxy is destructible. And not in some special super-weapons only way - everything has an HP bar, even the stars. You can move planets between systems. You can build deep space asteroid mining and metal refining facilities of your own design. You can build things with all their systems attached to the outside of the hull. You can design the tiniest of probes and the most massive of orbital stations. There's no limit to the size of ships, so you can build one the size of a planet. Or a star. Or a galaxy. The game won't stop you. It doesn't want to stop you. It wants you to do whatever you want on a literally astronomical scale.
Need I say more?