Played the original Civilization on an Amiga and have played every release since (including CivNet). Civilization II was easily the best game in the series. It's sad that Civ V, as pretty as it is, is so freaking horrible.
In my opinion II was the worst of the five. It was basically the same as the first with more of everything. More techs, more unit types et cetera. Didn't really add anything to the game except make it longer. It meant you couldn't finish a game in a single sitting which really lessened the appeal to me.
Starting with III they started added more dimensions like religion and culture. Sometimes it worked, sometimes it didn't but I appreciated exploring how strategies changed.
Edit: did a really poor job of reading your post or else you edited it. Oh well, just pretend my comment stands alone, please.