It's way too early to make a fair comparison.
So here I go:
1. Civilization 2 - A few oversimplicities (stacking) are easily redeemed by the sheer modability (near-perfect replay value), fast and easy gameplay in every era, attractive soundtrack, beautiful wonder movies, humorously quirky Council of Advisors ("Sir! The people! They can't help falling in love with you."). Notable for making huge improvements on the first game without dropping anything good, making it quite possibly the greatest sequel of all time, and easily in my Top Five Video Games of All Time.
2. Civilization - Dated beyond the point of playability, sadly, but for its day, it was a groundbreaking and horribly addicting game. It was my Tetris, really.
3. Civilization 3 - A sizable step down from the second, mostly due to the lack of non-map editing. Despite addition of resources, actually "boasted" simpler gameplay than its predecessor. AI completely unreasonable with its trades (to the point where it was hurting itself sometimes), at least until add-ons fixed them a little bit. Very limited early game, as research and modernization proceeds at a breakneck pace, with production able to keep up only during peacetime. Less Units, less techs, less options with the terrain all bad ideas that made gameply far less interesting. Ability to strip-mine in any province unrealistic and rather ugly, but sadly necessary in order to keep up in early production. Dropping of wonder movies was a huge disappointment; Isaac Newton's College was actually an introduction to Vivaldi for me, once I figured out where the music had come from. Despite this, a somewhat addicting game, but clearly inferior to the much, MUCH more flexible Civ2.
4. Civilzation 4 - As I said above, it's too soon for a fair comparison, but still, right now I have to place Civ4 down here. It has some major problems to contend with, even beyond the technical issues we all know of. As I mentioned in another thread (and I'll keep it very brief here), there is no early game. At all. Unit production times, especially early on, are way too high, and need to be modded to restore the smooth gameplay of the earlier games. Religion system only useful when there are many more Civs than religions, otherwise everyone will pick their own religion, and that will be that. It adds little more to gameplay than did Civ3's Palace View. There'll be plenty of time to finish my exploration of the game, but the first impression was far from auspicious.