I love them all, and am actually eager to buy the latest one. But for the high point in the series, I'd have to pick Civ. II Test of Time.
The regular game had all the basics that Civ player love, plus the throne room bonuses. But what I really loved was the alternate game modes Sci Fi and Midgard. It was like three games in one. There was a lot of depth in those extra modes, specially the Midgard scenarios, with the multiple Z-levels and the scripted events.