For me it's about the fact that Civ I is designed as a single player game. This means it has a lot of things that could be exploited, but that you don't have to exploit. Later versions had all sorts of highly unfun mechanics designed to nerf strategies you might not even be playing, but that could hurt your freeform game greatly. For me the order of preference is I,II,IV,V,III. Largely because III had the most unfun corruption mechanic ever (and took forever to tune it to reasonable levels). Civ I also got me interested in programming and exploring how programs work. I remember hacking around with the credits to get it to display me and my friends names for a while (reinstalled it later to give real credit where credit is surely due). I think a few UI tweaks would be really nice, I hate that you can build a settler to destroy a city, ZoC would do better with tweaking, and borders + slightly better diplomacy engine + better AI would be great. I've never felt the improvements I most wanted to make to Civ were graphical, yet that seems to be where most of the work goes in newer versions.