Civilization IV is the best turn based strategy I've ever played. Perhaps with also HoMM3(WoG) and the serial of total war. But they can't be compared.
The fact that you can almost mod everything in the game is the reason that makes it the first one.
For the fans, check out the biggest community of civ IV modders : Civ IV community Fall from Heaven 2, the best mod i've ever played that turn the game into a fantasy world : Fall from Heaven 2 And my favorite map script ^^: Full of Resources