Personally I found:
Civ III aggravating (other empires think they have the right to send settlers through your territory even if you order them out repeatedly, and they removed ZOC so there's no effective way to stop them - I forget what else, if anything, but that was the most annoying thing)
IV boring after a couple games
V also boring, but far more quickly, and the multiplayer seemed broken (and takes forever to load, only to have a 30 second turn; worse, you often do nothing but click next turn; it also failed to even work, or fucked up horribly, because OH YOU DON'T ALL HAVE EXACTLY THE SAME DLCS)
II was good for the time but it's not the best game, really. It was easy to mod in new artwork and stuff, though, without a lot of skill, and there were a bunch of scenarios which didn't use stock units etc, if you got the expansions.
I don't really remember Civ I very well, but I know I played it because I remember the intro sequence.
SMAC/SMAX was the best Civ I've ever played, but the multiplayer had issues (I had an internet game end because someone decided to just stop playing, and there was no way to do anything about it, or continue the game without them, or turn their empire into an AI, etc)
Beyond Earth... I have not played because it sounded like it didn't live up to SMAC's legacy.
But today I would rather play Dominions 4 than any Civ.