SMAC is one of my all time favorites, and I still have an install directory I move around from computer to computer to play now and then. As another commenter

SMAC is one of my all time favorites, and I still have an install directory I move around from computer to computer to play now and then. As another commenter said, Stellaris is a great newer one. A bit rough on initial launch, but well shaped up after patches and dlc. The Endless X series is another one, but personally they never clicked for me, feeling oddly soulless and empty.