I'm a huge fan of the randomness of the series which gives it unlimited replay potential, without mandatory multiplayer.
You can play several paths to victory, you can take over the world via military might. You can forge an alliance and win diplomatically, you can create a culture that dominates the world, and you can win a scientific victory via building a space ship to alpha centauri. You also can play as the German Civ, the Japanese, the Chinese, the Americans (an anarchonism), etc....
There is no one way to play and no game is ever the same. There is also several stratas of difficulty so it can be played by relative novices or experts.