Civ IV: 9/10
Civ IV gets it's 9/10, because unlike most other Civs, you can have a good game that will last for as long as your computer can handle it. Unlike Civ V, it can be saved when there is no possible victory, or when there is one, none of the AI rush to get it and simply become a**-***s. You can also hold a sizeable military, as seeing in V you can only have one army per block, and even then the wars are sketchy, unorganized, drawn out, and just generally irritating. Even if you like them. In Civ IV, the global warming, collapse, large-scale nuclear war, and eventual reduction of humanity to small coastal cities, where everything inland is nuclear wastes is fun. I love ruling an Empire through its birth, growth, adulthood, and even until the AI bests me, or the world is so uninhabitable, no one wants to fight. To me, it's only flaw is the lack of the ability to use your workers to rebuild the land and terraform, this would make recovering from nuclear wars of simply natural global warming, much easier, and not an unseen process. (I've only played until most of the world was desert, I'm unsure if the planet actually fixes itself.)
All in all, I love it, and have logged six months of play-time.