This poll is a little odd, but I suppose it was inevitable. After all, without complaints about AI, this forum would have significantly fewer posts even going right back to Civ III
The question should really be "Which Civ game had the AI that is the best at playing that Civ game?".
To which the answer is probably
Civ IV BtS with Unofficial Patch. Civ V introduced a number of features, notably 1UPT, but also including more specialised leader abilities, that are a lot harder to deal with than stacks and simple +/- modifiers to certain attributes like expansiveness or whatever. It's not like the AI itself has actually regressed, just the tasks it has been given are notably more complicated.
So not only did BtS and this patch have a number of years to improve upon the similarly-derided state of the AI in Civ IV vanilla and Warlords, it's also playing a game more friendly to the computer player.
Civ V has had a number of years of modding, and with the Vox Populi/Community Balance Patch is an improved state, but the AI is never going to be as good at the basic mechanics as it was in Civ IV.
Civ VI has only just been released, and has similar mechanics and specialisations that are harder to program AI for. Subjectively, I feel it's in a better place than Civ V was at launch (which isn;t exactly high praise). It has serious issues, but the game is still fun. I have every confidence that the next years of patches, expansions and mods will improve the AI to a higher standard.