There may be graphically superior, technically complex and more advanced versions of this genre and even game. But I'm picking the one that has a secure place in my personal gaming hall of fame.
Civilization II + Conflicts in Civilization and Fantasic Worlds expansions - 1996 - PC
The game greatly improved on the premise set by the original (for example, no more chariots destroying battleships!) while keeping intact the relative simplicity as you guide your civilization from prehistory to space flight.
Although there are only two official ways to win (Conquer the World or reach Alpha Centuari) there is a seemingly endless variety of ways to play focusing on military, culture, science, economy or as Slim hinted as his favorite, out right genocide.
And while playing the game itself provides hours, days even weeks of entertainment, the map and senario editing feature is the real jewel.
On a personal note, on one dreary winter break (most of my friends were out of town), I managed to create a fairly accurate and detailed U.S. map (cities included) and a modern day "Second Civil War" senario pitting the East vs. the West.
And that was just a couple of weeks of boredom; imagine what I could conjure with a lifetime.