A sort of the sequel to Civilization 2, SMAC is the most beautiful science fiction story I've ever seen. Not to be taken lightly, Alpha Centauri deals with heavy existential themes on essence of being, technology, the best way to lead society, and the way you lead your colonists has a gamut of awesome and terrifying power: You can construct anything from the worst technological dystopia to a utopia where all untold human suffering has been worth it, with flavour text and lore being backed by believable precedents and roots in real physics and biology.

For a canonical playthrough and the best first impression, play as Gaia's Stepdaughters, and shoot for Transcendence Victory. Let it envelop you entirely.