In my experience, the AIs that perform best are the ones whose algorithms most recognise the value of conquest and expansion.
They are:
Pocatello, Alex, Dido, Monty, Bismarck, Oda, Casimir and Shaka.
Sejong can be a threat if he has natural protection from mountains or something like that, but in general, if they turtle they suck, because while the Tradition civs are reaping the benefits of their 3 cities, the ICSers are having lots of benefits with very little drawback.
BTW, this is on Deity.