I stumbled on Civ 2 after the copy of the original Command and Conquer, for reasons I never understood, came bundled with it. Throughly enjoyed it, and the original when I was given a copy of that as well.
Stopped with Civilization: Call to Power which really put me off the genre. I've recently learned that it wasn't made by the original developer and Activision had licenced the name, which probably explains why it wasn't a great game, or why it's not mentioned above.
That said, I also played Alpha Centauri which is hands down the best of the genre that I've played. The expansion pack played so differently but was still obviously part of the core game and is one of the best game add-ons I've played. And the whole thing is available on GOG for less $2 as I type this post, definitely worth checking out.