I will never tire of Civ4 and ROM: AND (Rise of Mankind: A New Dawn otherwise known as that mod Tolni so shamelessly plugged in) is the main reason why. How do I explain it? Well, if you are familiar with Caveman to Cosmos, then imagine it a little like this- Caveman to Cosmos is a fat man you've met who turned out to initially be interesting, but as you got to know him better (ie halfway through a game) you realized he was pretty boring and all his topics of conversation sounded the same,and that sometimes he didn't make sense. AND is that same fat man trimmed down to a perfect weight with additional charisma and intelligence and personality added on top. Know you are talking to someone who can hold a conversation and your interest, and only rarely speaks out of his rectum. Suddenly, he's your best friend (ie you've played eighty hours, and are still planning on playing more).
Basically, AND is a hyper focused Caveman to Cosmos that starts from 6000 BCE, and is constantly fixing bugs and game play issues.