AI ruins part of the fun for me. Here is my take on the issue.
The problem with games such as this is that in the "difficulty" department, it appears as if developers just don't bother trying to make a better AI, they just give options for x5 or x10 stats and other unfair advantages. This is fundamentally counterproductive action. What they should instead do, is let the game fair and actually do something to improve the performance of the AI.
There are some folks who do this right or are on the right track. A good example is the dude that uses Fritz chess engine to code the Highly superior AI that doesn't cheat for Homm5.
It is well known that chess engines have now passed Super-GM level. Human GMs will lose to them and so Chess AI shows that it is indeed possible to create an astonishing well made AI for a strategy game that is TBS.
If chess engines can not be used to implement the calculation of an AI for a game such as Civilization, then there are other steps that can be done, but only if there exists the will for it. Get the right Building Orders, set the right anti-manipulation defenses, etc (it would really be extremely easy to make AI always refuse to give money for useless Open Borders, wouldn't it? Yes. They make it? No? Why? Your guess is as good as mine, but there's 2 possible answers, they don't care or they're just incompetent.