I'll go with C3, with C2 a strong 2nd. C4 was simply going through the motions for me with little fun. I found the AI as easy as ever once you learn it. My last game I was well ahead in tech, etc. Space race disabled, hoping the AI would get meaner...eh...nope.
Don't remember the year but only had 10 or so techs left to research and yet the AI has never delared war on me. Despite trying to bait it into doing so by having one defender in all cities, most of these long out dated units, harrassing the AI only gets and keeps them annoyed with me but that seems to be as far as they'll go. Also have a two or three civs on friendly terms.
And by the way, I'm not one who simply likes to war all the time...50/50 I am.
In C3 even on lower levels the AI would declare war, even it it was stupid at times for them to do so, it was still loads more fun.
The other day I spent an hour or so playing C2, first time in years, been so long that at times I know not what I'm doing. But, the little time spent on it gave me that good feeling again. Last night I loaded a saved game (C4) and got my first CTD, I reloaded, moved a couple of units...stared at the screen and relized I'm just going through the motions , not enjoying this game, whatever the reason is, I'm not getting from it what I did and do from C2 and C3.
So for now I've uninstalled C4, already have C2/C3 on the pc and will give C3 ago, although this means I'll have to set certain restrictions etc. on myself to make up for the AI, I'll know I'll have the fun factor again. Oh, I pick C3 over C2 mainly for borders, resourses, etc.
My rig is P4 3.6Ghz
3Gigs of dual channel ram
BFG Nvidia 7800 GTX
No spyware, adware, viruses, etc............
and at times this game......well there is no excuse for this game (C4) doing what it does to some machines!
I would like to point out that I'm not hating on C4 and more importantly not Firaxis, I love the Civ series and am happy for those who enjoy the game (I envy you) and hoping that with an ExP, that this game will bring me what the others have/do.
In the meantime C2 and C3....disks are at the ready........