Civ IV for me is the definite version of Civlization. The game as lots of small details which make it look like they really wanted to make an awesome game instead of just a profitable product.

Example: When you build a World Wonder, there is this little CGI move showing the wonder being build. There was no need to make it, they could just put a picture of it like Civ V and it won't really affect the sales.

Why they went the extra mile? Because, in my mind, they cared. The game is full of this kind of small details. Civ V looks bare for me in comparision.