The nuclear missile.
When I first got into the series with CivII, the first game I played, I didn't do much beyond building a few cities and some improvements here or there, without really knowing what those improvements did. I was pre-occupied with going straight for Nukes, and I was quite gratified when I built my first nuke and leveled a French city with it. I wound up building several more and using them. Then the polar icecaps melted and all my cities became swamped(literally).
It was afterwards that I seriously attempted to play the game.