Two ratings:
What I would play today:
IV -> III -> SMAC -> Col -> I -> II
In comparison to their time:
I > IV > III > SMAC > Col > II
Civ 1: Absolutely a revelation. There had never been anything like it. Yes, the AI was dumb beyond belief and games took forever and there were a zillion ways to break the game but who cared? The atmosphere was so cool, the choices so varied and the background was just so evocative.
Civ II: Just Civ I with a few things added that made a slow game even slower. Oh, and some new graphics that required me to get a new machine. Somehow, the AI seemed *even dumber* than the Civ I AI, which I wouldn't have thought possible. OK, the advisors were funny the first time I played - but not as funny as the Civ 1 commie advisors! I spent less time than on any other Civ.
Colonization - pretty fun, and different, but Indian trading and especially the War of Independence redefined the word "tedium". Fun if you stopped about halfway through.
SMAC - very inventive and evocative but had an AI functionally *even worse* than Civ II. Better actually, but the game was so much more complex it was even more lost. But the storyline was different, and I enjoyed playing with all the new ideas. Later I enjoyed modding them out so the AI could play
Civ III - the first Civ which was really a game, in the sense that it was possible to lose other than at a ludicrous difficulty. It was so much fun to play against an AI that actually had some idea how to play the game! And playing games that weren't decided by the beginning of the Middle Ages! Loved culture flipping - conquering an enemy capitol no longer meant game over, you won. Lots of great ideas like culture and happiness resources that really added to the game.
Civ IV - by far the most polished; it's basically Civ III with all the wrinkles ironed out. Maintenence is a better solution that corruption. Love the modding, too. I wish conquest weren't quite so rewarding - too easy - but I am still quite content.