It's Civ - nuff said!
This is by far the best Civ game - I hated Civ III - I don't even know why, it just wasn't as good as Civ II and felt like a step backwards and Call to Power etc were non starters. Civ IV is as good as the original though, the game play seems better balanced and the whole thing works much better. The territory bit where you can't enter someone else's territory (and they can't enter yours) works well and I like the influence factor where competing edges of empires gradually alter each other, so if you are doing better culturally, or religiously you start eroding your neighbour's control over their borders - that seems far more realistic and works well. Culture works well and the introduction of great people does as well. The tile improvements system works really well and gives you far more to think about and makes different terrain far more interesting - you end up building cities in the desert and in the hills rather than concentrating purely on grassland, river tiles or coast - vineyards (what the hell is a 'winery' anyway??) and all the other improvements make for a great increase in depth and strategy - I also like the idea of cottages, growing into villages and towns and effectively expanding your city.
Combat seems to work much better (although in my most developed game I'm still only in the medieval period as I'm playing several large epic games) and the experience system is a nice improvement.
The whole feel of the game is far more balanced, detailed and accurate - there is less about it where you're wondering why on earth it just did that and it just has the natural feel about it that the original had. The new game engine as well is a massive improvement, but the pretty graphics don't detract from the game.
All in all I expect to lose a large chunk of the next few months playing this and I can't wait!