It may be nostalgia, but CivII hit a perfect mix that enthralled in a way that the future games never quite reached for me. I think Civ IV was the mechanical peak for the series, but CivII will always be the "vibe" peak.

The throne room, the live-action advisors, the wonder movies. Go watch CivII's wonder movies for the Oracle, the Great Lighthouse, the Manhattan Project, or the UN and tell me they didn't catch lightning in a bottle.

In a pre-wikipedia age the "Civilopedia" was also a huge deal for me as a kid. I read every entry over and over again. Civilization II gave me a context for my place in history that served as a ground work for a lot of my future learning.