Maybe it's because it was the first one I played, but something about the 2d aspect of this game gripped me. I don't have the same love for the games that came after it, the 3d graphics somehow take me out of the historical element.
The fact that in the older games you are basically playing on a map, gives off this almost board-game feeling that is cosy and immersive. (I know that civ was inspired by a board game)
Playing civ3 as a kid really made me feel like I was in whatever era I was playing at. Still remember those jazz fusion tracks especially.
I still have the ~300pg instruction book for it on my shelf. Still the only game instructions I've read cover to cover.
It had a lot more detail than I'd ever seen in a game booklet before.
Great game, and the last civ game I felt like I fully understood (even if I wasn't great at it).
Oh. And I remember my automated workers paving the entire world into roads by mid game.