My favorite was Civ 3.
The first time I played civ was civ 2 on a cousin's playstaion, but Civ 3 was the first one that I got to play until late in the night, late into the next night that is. At the time I was playing Starcraft and I remember getting the tech tree paper and thinking it was huge. Then I got Civ 3 and the tech tree just kept going and going, then there were three or four other trees that you had to pay attention too. I killed so many hours in all sorts of decisions, what to build, where to go, etc, etc, etc.