I have Civ4, which is my favorite because of its easiness in modding. You can always find user-created mods way better than the vanilla version. I played the demo for Civ5, but I didn't like it one bit. I think the main thing that killed it was that you couldn't stack units, and the city-states. It was really hard and expensive to get a city state to like you, so in the end they just became an AI that didn't play the game.
