I don't think it's due to the virtues of the game so much as to my own chronic addiction to it, but Alpha Centauri may rank alongside Arcanum in terms of hours of life spent. I used to love Civilisation (the original, then II and III) and my long-term goal was always to win the game by sending a colony ship to Alpha Centauri. Then, when the Alpha Centauri game came out, my young brain became hopelessly devoted to it even before I played - just on general principles. I mention it because I recently got the game again, and immediately remembered how much I loved it.
This is one of the games my friend always tells me about, and which I want to try in the near future.
I play Stars! because it's awesome and simpler. But yeah I played Alpha Centauri in the past and it is extremely fun.
Playin' a big map... S.M.A.C. my bitch up! I love that game too. In particular the way that your choice of social policy affects how the AI teams treat you. I don't remember seeing that in any other game. It beats the socks off the usual "give them presents to make them like you" way that diplomacy is approached in the 4X genre.