I lost many, many hours to Civ 1 as a boy. It was a rarity in the battle with my parents over seat time as they saw it as educational. Ha!
I've tried to get even remotely as engaged with the new versions but i just can't seem to bridge the gap. In the quest to make them more detailed an deeper/broader they've lost the fun for me. I'd love to see the original game-play, 100% intact and true to the original but in the modern engine for the visual treat. Dare to dream.
I think the second game was an improvement in gameplay over the first. IMHO games 1, 3 and 5 changed up the gameplay whereas 2, 4 and 6 refined the previous games. So if I had to choose one to have a "HD" upgrade, it'd be 2.
But of course this misses out Alpha Centuri which is a classic in its own right. I'd really like a modern take on that (no, Beyond Earth doesn't count).